Well, the Library of Congress has billions of tunes, a few of which I've learned (sprirituals). For the rest, I guess you'd have to do what the rest of us do:

1. Buy and listen to CDs (including Mudcatters' CDs which often have lots of learnable stuff, affordably)
2. Comb through old threads (by songtitle) to hear clips Mudcatters have found here and there online, and to which sound files they have cleverly provided links
3. Use libraries (3-D as well as virtual) to absorb as much free sound as possible-- a good example is the Digital Library of Appalachia
4. Carry an inexpensive MP3 recorder to concerts of artists whose repertoire you respect, as well as to Mudcat gatherings, jams, sessions, etc., to record snippets of tunes you'd like to learn (with their permision of course)
5. Play as much as you can, with other folks who like the genres you'd like to learn
6. Use and record streaming radio, links for which are also stored around this place somedamnwhar (I think there's a Permathread full of them); the US and the BBC have lots of great online music and online concerts archived

~Susan