There are many companies from whom you can buy domain names, usually for very reasonable yearly prices. A Google search for domain registrar will turn up numerous options. Keep in mind that you can also buy a domain name directly through Blogger; doing so will configure your DNS and Blogger settings automatically. You can also shop around through this list:
Once you've got a domain name registered, you can use it on your Blog*Spot blog. (Or buy a hosting service along with it, and publish via FTP.)