Amoeba Music ships anywhere in the US, but I don't think they sell secondhand CDs. There's also eBay, I suppose, as well as Amazon. I usually try Amoeba first, though.
Also...
=> Bandcamp: buy digital music (and sometimes physical media) directly from artists
Bandcamp doesn't do DRM, and you can download purchases as MP3, Ogg Vorbis, or FLAC.
Oh, this is definitely worth a look!