Cars can be purchased at the Boabo Auto Merchant. Complete the following missions to unlock the cars.

Car	How to unlock
500 XLR8	Get a bronze medal in all Algonquin races
Banshee	Complete the mission "Weapons of Mass Destruction"
Bulldozer	Complete the mission "Conterfeit Gangster"
Cityscape	Get a bronze medal in all time trials
Cognoscetti	Complete the mission "Grave Situation"
Comet	Complete the mission "Jackin Chan"
Coquette	Complete the mission "Raw Deal"
Formula R	Get a bronze medal in all street races
Go-Kart	Get a bronze medal in all Go-Kart time trials
Hearse	Complete the mission "Wheelman"
Hellenbach	Complete the mission "Bomb Disposal"
Infernus	Complete the mission "Cash & Burn"
Limo	Complete all of Guy's missions
MK GT9	Get a bronze medal in all Broker and Dukes races
NRG 900	Complete the mission "Wheelman"
Patriot	Complete the mission "Wheelman"
Resolution X	Complete the mission "Pimp His Ride"
Rhino	Complete all story missions
Sabre GT	Complete the mission "Operation Northwood"
Style SR	Get a bronze medal in all Bohan races