Might want to start at http://www.mandolincafe.com.

Mandolins are tuned GDAE, same as a violin. So any fiddle tune can also be played on a mandolin. You'll obviously play it differently though - you can't do the "lift" like you can with a bow, but you've got tremelo picking, full chords and rhythmic open strings to add into the mix instead. There is a "Mandolin player's Fakebook" containing tabbed transcriptions of fiddle tunes for mandolin, but it didn't seem much cop when I had a look through it. The "Fiddler's Fakebook" or "O'Neil's top 1000" (or whatever its name is) have more content, providing you can play from regular sheet music. If you can't, this is a good time to learn...

Elderly Instruments (in the US) or Hobgoblin (in the UK) should stock the "fakebook" and O'Neil's.

Graham.