Well, stealth cars were for EU signature edition (whole 6) and for preordering the game (just 1 or 2 mostly, but i actually got a whole 6 that way).
Chrome line cars were for buying collectors/signature edition, except jaguar (that one you can get on facebook, but for Europe only).
So, the people who got those codes (you just get codes for PSN and download those cars) selling them on ebay sometimes.
So either you buy some special edition of GT5, buy those codes from other people or ask a friend who has them already to active his account on your PS3 and re-download cars.
You can't unlock them just playing the game.