There are tons of registrars, just pick one. I recommend you register with someone other than your host. Don't pay more than 10 USD per year for a .com.
I've used
http://www.cheap-domainregistration.com/ in the past and have been very happy with them. They offer full control over your domain so you don't have to wait for their admins to set stuff up for you. I'm sure plenty of other registrars offer the same service as well.