There is no DRM support in MP3 so if you insist on using MP3 format you're limited to some of the more obscure "off label" stuff.
If you're open to using Fairlplay'd AAC then iTunes. If you're open to using WMA with Microsoft's DRM then I hear that Wal-Mart is pretty cheap.
There is also
http://www.allofmp3.com. Although they haven't been forced off the market yet, there seems to be significant doubt about their legality.