IMO, the highest quality music comes from obscure brick and mortar places called record stores :Q
If you live out-of-town or getting out of the house is an issue, websites like Amazon, CDNow, and SonicNet actually deliver!
Then if you want mp3's, you just rip the CD.
Otherwise (and on a serious note), you're just paying for lower-quality music and don't get the fancy extras, like liner notes and cover art.