if you are talking about the used cars (22b, 360, 240z, etc..) its random. most, if not all are in there pretty early in the game, when you dont have the money. the used cars change every 10 days, so you could just rack up the days or start a new game on another card and spend all the money buying these cars early. then buy them onto your normal card, which is what i did.
hopefully that helps.