Some local and nonlocal resources for chamber rmusicians:
(If you know of some others you think should be on the list, let me know.)

Amateur Chamber Music Plaayers (ACMP).
     The ACMP is an organization of amateur chamber  musicans that provides a number of valuable services to its members. The ACMP maintains a database  consisting of the musical profiles and contact information for members, organized by geogrraphidc region. Members can search for other players when forming ensembles and planning other group activities using a number of different search filters. They also provide an annual listing ofsummer chamber music workshops, grants providing partial payment for chamber music coaches,  and support for local play-in activities organized by members. Members pay an annual fee of $25.
Northampton Community Music Center(NCMC)
    A community-based nonprofit organization with a permanent faculty of instructors and coaches that staff a variety of musical programs. An adult chamber music program runs twice a year in which individual players are assigned to groups, prepare a musical selection under the direction of a chamber music coach and perform it in  a recital at the end of the  term. There is another program for 2- and 4-handed piano music.

Northampton/Amherst Chamber Music Concertgoers  Another area Meetup group of chamber music lovers that regularly meet for dinners at  restaurants before evening concerts and after concerts in the afternoon.

Apple Hill Chamber Music 
    A  nonprofit chamber music organization located on  a  campus several miles north of Keene NH with resident string quartet that organizes  five 10-day student chamber music workshops each summer.  Students are matched with musically compatible players (regardless of age) and  prepare assigned pieces from the chamber repertory for a recital at the eic, and of the session. Participants are placed in two coached ensembles, one of which will probably be close to one's technical limits. The second assignment is usually less challenging. Students must submit an audition recording in their application. The center will accept qualified students at all levels, from musicians just getting started with chamber music to conservatory students and professional musicians.  The playing-for-peace' program of the Apple Hill Quartet draws students from all over the world, and gives the workshops a uniquely diverse and stimulating character.

Stacey Styles Violin Restoration   This is where I go for  work on my instruments, both for routine adjustments and inspections and for larger and more complicated restorative work. Stacey is a highly skilled restorer and does all the work herself on the instruments that are brought to her.  Her shop is only about 20 minutes from Amherst if you go over the notch.

The Petrucci Library-IMSLP An online repository of pdf files consisting of scans of sheet music in the public domain (both individual parts and scores. )  The project is similar in spirit (and was probably inspired by) the open source code environment  for the the Linux operating system.  Since anyone can legally scan and disseminate sheet music in the public domain without violating the rights of composers or their estates , the IMSLP repository was developed by individuals around the world contributing   scans of out-of-copyright sheet music, usually borrowed from music libraries to the IMSLP collection.  Copyright law is very tricky here, since the publishers and composers come from all over the globe. Generally, this means you're only going to find music there dating. from the very early 1900's and before. Companies also sell CD's with scans of such music -- I think many of them simply downloaded music from IMSLP then put their own logo and copyright restrictions on their versions, then sell them.
      For quite a while after the project started, their collection was too incomplete to be of much use, and the scans were frequently of poor quality or in color and didn't print out well. In recent years, the size and scope has increased exponentially, and it is quite likely you'll find scans of the scores and individual parts of many compositions  of the most important pre-1900 composers, frequently with high  quality scans that print out well and can be used for playing music. They also have a huge collection of scores by lesser composers.  Look for black-and-white scans: the files are much smaller and print out the best. 
     This is a great place to scores for string quartets etc, where there's no piano part. Its also very useful for searching for repetoire for a particular group of instruments where you're unfamiliar with the literature.  For example, if you want to search for music for a flute, violin and cello, under their "browse scores" menu item, go to "Instruments/Genre", then "trios", then "subgenres",  then either  "flute, violin cello" or "flute, violin cello (arr)".
       In case you were wondering,  the name "Petrucci"  is in honor of  Ottaviano Petrucci, who in 1501, created  the first press-printed book of polyphonic music.
The UMass/5 College Library  
The combined sheet music collection of the 5 colleges in the area is very good, and you'll frequently be able to find what you're looking for. Students, faculty and  staff of each school can search the collection online. I  gave the UMass link above, and you can get to the library links to the other schools from the UMass link.) You can also borrow (from the UMass library at least) by going to the library and showing them a library card for a town library.  You can also probably borrow a music score  from  the 5-college college libraries from your  town library through the inter-library loan system (Iliad.) Saves you a trip to one of to one of the colleges but takes longer to get.  You can also get sometimes gets scans of music that none of the local libraries own through Iliad, but some nonlocal library must be willing to do the scanning, so this probably only works for short pieces.
Midi Renderings and play-alongs:
    I've been using computer-based midi renderings of chamber music to learn the cello parts of new pieces quite a bit, and have found it  be a very valuable tool. It ain't music, but its a great way to learn how your part fits together with all the other parts. You can also try to play along with a CD after you know  a piece,  but the correct tempo is far too fast in most cases while you're still learning the piece, (and sometimes, even after you've learned it!) The great thing about midi renderings is that you can set the tempo to be as slow or fast as you want without degrading the audio quality. Programs exist that slow down or speed up audio files of recorded music,  but the audio quality becomes bad enough to make it useless when you slow it down more than 5 to 10%. Also, with midi files, you can omit any track you like, or make it softer, so you can play along with the computer in a music-minus-one format. Another good feature of midi is that you can change the instruments to whatever you want. I usually switch the violin and viola tracks to clarinet, which sounds much better to my ear than than those dreadful hurdy-gurdy cartoons of how string instruments sound that you get with midi.  A midi bassoon doesn't sound much better than a miti cello. Midi does the best with its rendering of piano parts.

    Here are some free resources that I've found useful, but its just the tip of the iceberg:

Finding midi files on the internet: Suppose I'm looking for  a midi file for  specific piece, e.g. a Beethoven string quartet, I'll search on google with something like " beethoven quartet midi", or even "beethoven midi", then go to a midi site  that focuses on classical music or on the music of Beethoven. Most of links will be junk, but there several repositories that give you a limited number of  free midi downloads. Chamber music is usually not a high priority item for the larger sites, but you can find a lot this way. 

Midi repositories:
the list  is endless, but most it is garbage. Here are a several that several that are decent:
HarfeSoft.  A very  nice selection of free midi files for chamber music (including violin-piano, cello-piano) duos of the big names from the classical and romantic periods. This is someone's pet project, not a commercial one. The number of selections is very limited, but whoever did it had good taste.
Kunst der Fuge A commercial midi file repositoary, but if you  register (no big deal) they give you 5 free downloads per day. The have a pretty good choice of chamber music midis for the most important composers. 

midi software: players,  editors, and services PC"s and macs will both play midi files if you click on them, but with very  little editing ability. The freeware version of Quicktime (on both macs and PC's) has a slider that lets how change the tempo arbirarily, but it is not calibrated to metronome settings.  You can't delete or  alter individual tracks as far as know, or change the instruments.
Aria Maestosa: a free, open source midi player. It was developed as a Linux program but has free downloads for  Windows and and Mac versions. The editor let you delete tracks and change the instrument, but the tempo editor seems to have a bug and doesn't work (for me, at least.)You have to import midi files. You can then export the modified midi and play it with Quicktime, where you can change the tempo.
Rosegarden An open source (Linux)  midi and audio sequencer that provides an enviorment for generating and editing (printable) musical scores from midi files and for editing midi files. You can  use Rosegarden for simple changes in tempo,  for changing the key and transpositions by intervals, and many other sophisticated things.  The music score editor lets you write your own notations into the music. You can then print out the midi file as a pdf file for a  musical score.  The score editor is based on LilyPond which is an open source music engraving program based on the Tex and LaTex programs for typesetting mathematical text. If you're familiar  with Tex or LaTex, its easy to learn to write LilyPond code. This allows you to write your own music using only the commands typed at the keyboard. Rosegarden is only available in Linux. Is best to install it through an installation program provided by your distribution rather than getting it directly from the Rosegarden, since the program has to fit in correctly  into your system to find the libraries and devices it needs to run properly. I'm not sure its available with all distros; Ubuntu is one that supports it and makes it available.
Naxos sheet music service: a propriatary service which has a large number of digital chamber music (and other) scores that you can play as midi files and print out as sheet music, with midi editing capabilities similar to those of Rosegarden. Faculty, staff and students can get access to the Naxos sheet music library (and also, to the Naxos  library of recorded music) through the UMass library's  database page for music  , but you need a a valid UMass library user account . Both services can obtained by the public by purchasing subscriptions from Naxos. Their recordings library is huge. There's never be a piece I couldn't  find there. Their sheet music library is large, but finding particular pieces there can still be hit-or-miss.

Another resource:  A website where a professional accompanist, Paul Gardner (no relation) sells recordings of piano-only recordings of his accompaniments to a large number of pieces from the  string, woodwind, and birass solo repertory. Gardner has a Masters in Music Performance, and his accompaniment files are about  the best of this sort of music-minus-one format that I've come across. He uses a high quality digital piano to make the recordings  They can be fun as well as instructive to play along with. You can't change the tempo without sacrificing audio quality, but he does provide practice versions of faster movements.  Both the Schumann and Beethoven sonata recordings in the recordings link in my profile were made with his accompaniment files playing through speakers while I was playing the cello. Not bad, but still no substitute for a live pianist playing a real piano.