Codes can come from many places. This site has supplied at least one code that I know of. The other codes came from buying the original Collectors and Signature Editions of the game. Still other codes come from third-party places, like Edge, for the Edge Camaro SS or GameStop for certain Stealth cars and the NASCAR #20 Joey Logano Camry, to name a couple.
Ebay is also a place to purchase unused codes, but be wary of scams. Sometimes they sell already redeemed or expired codes.
As you are in NA, and if you don't have it already, here is a code for a Stealth GT-R GT500 that you can enter in the PS Store to receive the car into your Garage:
RD69-FJN9-5N6Q
Just FYI, but it expires on January 30th, 2012. 👍