Used CD stores in Japan. Har har.
But seriously, how much is "cheap enough"? You'd be hard-pressed to find a copy of these for less than retail price, unless you search around on various gaming soundtrack sites for someone willing to sell you their used copy. Oh, and of course, be very, very careful not to buy a bootleg. The publishers you're looking for should either be DigiCube or Square-Enix when shopping for a legit copy.