brad recently recommended the Amazon-mp3 store, and I have to admit, it's pretty decent. 256kbps, no DRM, $0.89 or $0.99, their downloader downloads to the standard file structure ("Artist\Album\Song") in a subdirectory of your choosing, so you can make it all one-click happy just like iTunes.
It turns out there are tons of old 70s songs that I'll pay $1 for that I would never buy the whole album of after all. Trolling through greatest hits for prominent artists seems to work well for me; I was downloading Clapton's Layla from a Clapton's-greatest-hits sort of thing (rather than from the Derek & the Dominoes album it came from), and I went ahead and listened to all of the other songs on it just in case, and sure enough, "Promises" was a "whoa, I remember this song from 25 years ago or whatever, I had no idea that's who it was either".
Unfortunately, there are also gaps, but whatever.