For a great collection of downloadable mp3s, try here:

http://www.juneberry78s.com/

Especially good if you're interested in blues and gospel. You'll need to request a password for the very large B&G listening room (about 70 CDs' worth). Instructions for that are on the page. The mp3s here are 32KBS and mostly run 580-710K in size. The proprietor requests that you not download more than 30 tracks a day. Not a problem for me, since I'm on dialup. Considering that most of the tracks are from 78rpm, the sound is quite good, as good overall as the reissue LP records I stocked up on in the 70s.

Also, http://www.publicdomain4u.com/
which has some overlaps with archive.org and juneberry78s.com, but also a few nonduplicative items. Files run about the same size here as they do at archive.org, 3MB or so for a 3-min 78rpm side.

Much less "folky," but this site http://cylinders.library.ucsb.edu/ has a large collection of old cylinder and Edison recordings up to c1925. Files are fairly large. Contents include pop, minstrel (many of them quite objectionable to modern ears, if you're looking for repertoire -- do a keyword search for "coon," though, and see what I mean!), a bit of jazz, and even some early Hawaiian recordings, which are what first attracted me to the site. Some similar material is on archive.org.