I'm going to point out the website for the IMSLP. Petrucci Music Library...
http://imslp.org
...which is a great source for public-domain music.
I've used various viola works and studies in alto clef, and cello works and studies in bass clef, to work on reading those clefs for CGDA tuning. There are even tenor banjo materials there in treble clef which are usable for mandola.
Good luck!
Bookmarks