I don't know any sheet music sites first hand, but you might check out this link:

Bluegrass & Swing Music Magazine
This site has an archive of live jam sessions with some of bluegrass music's best fiddlers, some of the field recordings go back as far as 1954. There are also some tabs for some of the tunes and I believe a links page. If nothing else, e-mail Jim Moss who runs the site...a great fiddler in his own right who loves the old-timey traditions and would be glad to point you in the right direction if he can!