Can you buy a NASCAR race car?

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.