Before you start the game, and if you have the latest version of the PSP update, go to the Playstation Store. While you're there, go to Add-ons and search Gran Turismo. You should find only one add-on that says: Gran Turismo PSP Car Pack. Download it for free, and in Gran Turismo when you start it should say five gift cars have been added to your garage. There you go- Lamborghini Countach LP400 (SP model) '74, GT by Citroen (SP model) '08, Nissan GT-R SpecV (SP model) '09, Enzo Ferrari (SP model) '02, and Bugatti Veyron 16.4 (SP model) '09.
After every 70 days, the dealership cycle starts over again. For example, day 71-72 will have the same dealership cars as day 1-2. So if you wanted a Bugatti Veyron, which shows up on days 45-46, you would need to wait until day 115 until you can have a chance to buy it again.