Back when commercial radio was more useful to me, I did find new old songs on 2 syndicated shows:
-Reelin' in the Years with George Taylor Morris (if you live in Boston, you may recognize the name, as he was on WZLX and WBOS in recent years, more on that in a moment)
-Flashback with Bill St. James
The latter is still on. The former is apparently not, as George now programs for XM satellite radio. I doubt it's worth my while to get XM for this but if I did I think I'd get it for home and use it as a source to encode from.
Interestingly I used to record both from an internet feed after everyone here stopped carrying them; The dummy audio driver in sparclinux was written to I could make realplayer write to it, and then I captured from that. At some point my scripts broke and I gave up, but I have mp3 encoded captures from 1998 or so somewhere.
Maybe I should start recording a feed of Flashback again.