The Internet Archive has digitized 28,000 78 RPM records and posted them online. Amazing. And they didn’t just do a batch conversion. Most songs were recorded with multiple needles, and an audio engineer picked out the best recording. They also used software to clean up the tracks further. So these are going to be just about as good as you can possibly get.
And they are all downloadable. As far as I can tell, these are all freely available to the public with no restrictions.
I did a couple of spot checks just for fun:
Sister Rosetta Tharpe – 8 recordings of 6 different songs, including “Shout, Sister, Shout!”
Hank Snow – 26 recordings! Including “The Rhumba Boogie”
Josh White – 6 recordings, including a recording of “House Of The Rising Sun”
It’s kinda hit or miss, but wow. what’s in here is a treasure trove!