A Different Drum - independent synthpop label and retail outlet
Amazon.com - the big one - be sure to check out the MP3 download section as there are many good, free MP3s available there from independent artists
Download.com - not as much a store, since the MP3s available here all free. But you can find a good variety of material from independent artists with a little work.
EIL - UK based collector's shop - carry a lot of material that is out of print or rare or both
Garageband.com - like download.com, a site for free downloads of independent artists. Not all tracks you find can be downloaded, though.
iTunes - the biggest of the digital music sellers. A fair number of exclusive tracks are available here - including a number not available elsewhere, or from now out of print CDs.
Silver Platters - a local store in our area, and one that we frequent fairly regularly
SecondSpin - specializes in used CDs - fairly good for tracking down out of print items when other avenues fail