Discogs is by far the best place to buy CDs -- especially out of print titles. But you will find new discs, as well from sellers such as C.C.Music.
Ebay can be good, although in my experience grading is not as reliable. Same goes for Amazon Marketplace. I guess it depends upon how picky you are about condition. Most of the carelessly graded discs I’ve had to return have been bought from these two sites.