The official naming of stars and other celestial objects (asteroids, comets, etc.) is done by the International Astronomical Union. The IAU does not provide commercial services on naming astronomical objects. Therefore, refer to a detailed discussion of this and related issues on the IAU web document about naming stars.

There are, however, star-naming companies that can, for instance, be found on the web (we do not list any of them here). Just remember that no matter how official they try to make their procedure sound, it's no more official than if you printed up a certificate yourself. If you still wish to purchase a star, check around and see if you can find a planetarium or museum or astronomy club which is selling stars as a fundraiser. That way, your money is likely to be going to some worthy cause.