Where can one buy Guitar Hero for Xbox?

You can't. Originally, Guitar Hero was developed only for the PS2. With it's second incarnation, Guitar Hero II, it sprouted wings and crossed over to the Xbox 360 as well as the PS2. In the most recent version, Guitar Hero III, the platform has been raised to include not only the PS2 and Xbox, but also the Wii and PC (as well as Nintendo DS and a mobile phone version).