Just found Run-DMC at emusic.com. Emusic currently has the latest Run-DMC relase Live at Montreux 2001 available.
Well, I doubt that emusic will sell other Run-DMC releases soon, as Arista (Sony-BMG) does not seem to be willing to sell their music DRM free for the time being.

Run-DMC – Live at Montreux 2001 at emusic.com