a lot of performance shops will build a car to show off to customers and whoever else, then when they are done with it and move on to a new project they will sell it.
i don't know of any companies that just sell cars they have modified, but their are a couple who will import cars from japan, motorex being the most widely known.
i just found this place in florida, don't know the quality of work, but you take one of the cars they have and tell them every part you want on it, from intakes to motorswaps
www.xatracing.com