I'm a fellow newcomer (though slightly less new, by the sound of it). I introduced a thread on this topic a few weeks ago. I agree that parts are a great idea, since I love to print out the music and try to play it.
Unfortunately, many of the scores are from "pocket scores" or "study scores," so the uploaders do not actually have the parts. Also, if you have a whole lot of free time and Adobe Photoshop, one alternative is to copy and paste lines from the score to make your own (in your case, viola) part. This is a huge pain, but it does make a part that is at least playable.
Also, I've noticed lately that many of the recently uploaded sonatas and concerti, as well as some trios, have individual solo parts. Hopefully, this trend will continue!