Paul, look at this address; it's a search engine that finds midi files, abc, and sheet music (gif) for tunes. It may be biased towards Celtic tunes; that's what I use it for. If you are also interested in Irish/Atlantic type tunes, let me know and I'll suggest more web pages that have lists to browse through. Sorry, I don't know much about the old-time stuff.ps sorry I can't do this as a link - I have a French keyboard that won't do the pointy bracket things.
good luck,
Marion