I dont see why its an issue? Most parts and components needed to build the vehicles are incredibly easy to find and salvage. Springs, Pipes, Leather, Duct Tape are widely available, not only in car wrecks themselves but also in furniture and trash.
As for specific parts directly related to motor vehicles, such as Engines, Batteries, Headlights, you directly get them from wrecks and yes, even in garages. In the largest Pass N Gas POI, you have at least 4 cars, one car engine inside the dual door garage that gives an Engine 100% of the time.
What exactly is the problem here?