while not really an mp3 site
LaLa.com
lets you trade your physical audio disks for other audio disks. $1.75 per trade including shipping. They provide the shipping supplies. so if there are more than 2 songs on a disk you would like, it may be cost efficient to get them that way. The rarer or more demand a disk you want, the harder it is to get. I have had no real problems getting classic rock and greatest hits collections.

I'm having lots of fun getting new disks.

Axiom M80, Ep600, Qs8, VP150, Crown XLI 2500 , DBX Driverack PA2, Focusrite Scarlet 2i4