Like Greg mentioned, the vast majority of aftermarket vendors purchase their own vehicles. In many cases they have customers supply donor vehicles. The aftermarket company gets a vehicle to test, and the owner usually gets free (or greatly discounted) parts and service in return. This is often the way to go for a small outfit with limited funds.
The only aftermarket companies that I am aware of that receive free (or severely discounted) vehicles are large integrators such as tow-truck, limousine, and fleet vehicle modifiers. In those limited cases, it makes sense for the bean counters to hand over the keys.
Good Luck
Bryan Carter