The best deals and finds will probably be found in the Used Car Dealership. Your first car will probably be a Used Car. Used cars can range from under 2,000 to 4 million credits, all of varying mileage with a pre-determined color. It is recommended that an oil change must be performed in GT Auto, with a rigidity refresher in the tuning shops. The cars can vary in age from the 1950's to modern cars, some even with little to no mileage (e.g. Bugatti Veyron 16.4 '09, Ford GT '02) making the UCD and OCD the only way to get most Standard Cars in GT5.
This mechanic originated in the original Gran Turismo, with the presence of local Used Car Dealerships (every manufacturer may or may not include a UCD). This is useful for finding cars that are not purchasable in the New Car Dealership (e.g. Nissan SILVIA K's (S13) '88, Mazda SAVANNA RX-7 INFINI III (FC) '90) especially when a race event involves cars made before a specific decade.
In Gran Turismo 3: A-SpecEdit
Gran Turismo 3: A-Spec discards the Used Car Dealership and allows the player to buy old cars directly from the manufacturer's dealership.
In Gran Turismo 4Edit
The Used Car Dealership returns, but this time it is split into three categories: Historic (for pre-1990 cars), Early 90's (cars from 1990-1995), and Late 90's (cars from 1996 to 2002). This is useful for racing in the Japanese 70's Classics, Japanese 80's Festival, or Japanese 90's Challenge. Oddly enough, the game treats these three dealerships as Used Car Showrooms.
In Gran Turismo 6Edit
Gran Turismo 6 discards both the Used and Online Car Dealerships and, for the first time since Gran Turismo 3: A-Spec, allows the player to buy simplified cars directly from the manufacturer's dealership or the RCD (Recommended Car Dealership).
The interiors in these simplified cars are all black in the game, with the exception of open-wheel cars, which have visible cockpits. The only downside to these simplified cars is that they have no gallery view.