The site www.lastrestmusic.com has GREAT books of MANY trios. You would need to get the Music for Three (obviously) - but if you want to stretch your possibilities, get the PIANO PART, too - that way, you can use a variety of the trio parts with OR without piano -and WITH piano, you can do duet, trio, or quartet - they ALWAYS work. These are not cheap . . .but a bargain. Perhaps this set (no piano) will work for you:
http://www.lastresortmusic.com/index.php?option=com_content&task=view&id=320&Itemid=42
- which is one of the INTERMEDIATE Music for Three, or you can get the original (adult?)the Music for Three that I already mentioned. A set of trio books will run you about $50 - will your SCHOOL spring for this? Yes, there is a VIOLA book!
For easy things without piano, look for the Mixed Bag series from Chester Music. They have 3 parts - again, your violist might have to re-do in the alto clef.
This is SO cool that you are doing this! Have fun! I play chamber music as a big portion of my living - it is SO much fun, and I make $$$ doing it! If ONE of you pays for the books, they should keep them and take them to college - betcha you get $$ work off these. I own many volumes like this, and they PRINT money for us.