First, there is no such thing as a fiddle tune, really. Anyone could take a tune and play it on piano, recorder, flute, dulcimer, or tuba. So don't look for fiddle tunes, just look for good tunes.

Start with Richard Robinson's Tune Book. It has links to other sites.

Or you could just buy the Fiddler's Fakebook (alternative title: 50,000 Eighth Notes) for $25 and save yourself a lot of time.