You can check if your desired domain name(s) are available on the Buy > Domain Name Checker.  As mentioned on that page there are some domain name guidelines to follow:

• Use only letters, numbers, or hyphens ("-")
• Cannot begin or end with a hyphen
• Should have less than 26 characters for best compatibility/access; must have less than 63 characters, not including the domain name extension (.com, .net and .org, etc.)

You can buy your own domain name from the Buy > Hosting & Domains .