I know there's probably stuff in the pile of viola things at home, but I would probably need to go have a dig through the pile before I remember any of them.
Just off the top of my head in the meantime, how about Vivaldi sonatas (I think there's a set of six of them which are transcribed from his cello sonatas)?
Handel sonatas (transcriptions of the violin ones) are good too.
There are also the Bach gamba sonatas, but they might be a small bit tricky. You could have a look at them though to see how you find them.
I do remember someone telling me about some nice baroque stuff in some of the later Suzuki books (about book 3 or 4 I think), but I think those would be in the violin books and I have no idea whether the viola books have the same repertoire. Again they might be worth looking into
Actually, if you haven't started the Bach cello suites yet, a good place to start might be the two Bourreés from the first suite in G