I found this website the other day: http://imslp.org/wiki/Main_Page
Here you can download the cellosuites of J S Bach, Arban's method for the cornet, Debussy's Syrinx and alot of other works that is suitable for the euphonium. You have to read about copyright on the site for yourself before you download. Bach for example is free of charge since he died in 1750.