I came along with the Kinsey Studies. The first set is easy - as you already play, the second set might be appropriate. They deal with technique up to about Grade 5 (ABRSM) - varied bowing, spiccato, staccato, martellato, chromatics, trills etc. The final set takes you up to 5th position (which I find uncomfortable on the lower strings!) and 4 sharps and flats. Some of them are quite musical as well being studies.

http://www.musicroom.com/Search/gsea...t=kinsey+viola

Playing in an orchestra is great experience.