Just buy them, lol. By creating a second account, saving some money (20 Millions will do, to get the most expensive ones) and then backing up your save before you buy your car and send it to your second account). Restore your backed up save and you still have the money, send the car back from second to first account and that's it, you can even backup your save from your second account before you send the car back and then restore it to maximize profits.