Well, it depends on what you look for. If you are looking for rarities and stuff like Ahat, Impulse, or any other LPs, forget it.
If you are okay with new releases and occasional used goodies - all western artists, nothing rare mind you, then you should go to S.M.F. and 0.4.3 (this reads "O-Che-Ze")
S.M.F. website: http://www.smf-bg.com/en
it shows a lot of stock, but I'm not sure if they have it on hands, or it is on order basis.
0.4.3. website: http://z043.org/
(in Bulgarian only)
You can also go to "Ploshtad Slaveikov" - there you can find plenty of Russian bootleg cds