If you would like to purchase a new domain name you can do so at any time from your cPanel account.
Simply select Domain Registration from the Optional Account Addons section and then enter your domain name. Each domain costs $14.95 per year.
Once your domain is registered you will need to add the new domain name as a parked or addon domain in your control panel. You can access you control panel by visiting www.yourdomain.com/cpanel and selecting 'Parked Domains' from the 'Domains' menu.