Domain names can be purchased at any accredited domain registrar. A domain registrar is a company whose main purpose is to register domain names for people.
Most hosting providers can also obtain a domain name for you, but domains can be more expensive this way, due to the added overhead.
There are several very popular domain registrars, including GoDaddy.com, Enom.com, and Dotster.com (shown here). Most of their prices should be about the same.
All three of these sites do, of course, have a way for you to check if a domain name is available for purchase. Let's take a look at GoDaddy.com.
We can enter a domain here, right on the home page
As we can see, the domain we entered isn't available. Note the list of suggestions below
This is the end of the tutorial. You should now know where to buy a domain name