Can you buy a NASCAR race car?

Answer 1

Yes. There are several racing series that use older NASCAR race cars, so they are available for sale. They are very expensive.

If what you really mean is "can I buy a NASCAR and drive it on the street," the answer is no. It would cost more to make a stock car street-legal than it would to buy a brand-new street car that your favorite driver's car is patterned after and have it decorated like a stock car at a body shop. People do that.