Dear Flint and Jeff: As you suggest, Jeff, Radio Spirits, which took over the vast Radio Yesteryear reel-to-reel library, has many of Welles' Radio shows on cassette, at reasonable prices. Some of the rarer ones can be dug out, custom, for a few dollars more. They are transferring everything gradually to CD, but I have found that the cassettes are more reliable than the discs, which sometimes won't track.
A point which I keep emphasizing is that these shows were broadcast for reception on heavy bass speakers (one reason Welles was so much in demand in Radio), and on tape or recordings, they tend to sound thin. Crank up the bass a bit for something more like the original experience.
Good luck on your adventure, Flint, and Happy New Year to all.