In the 1977 Era, go out of your safe house, then turn right. Then keep on straight until you find an alley way. In it you can find a place where you can customize your car with new wheels, body kit, side skirts, front, bumper, etc. Note: It is not available in the 2006 Era.
To customize your cars for less, successfully complete the game then choose the car or bike store at Rays. If it is for the 2006 era go to the 1977 era then customize it for ten times less money.