I don't know of an easy way to find out this information out, unless you can find a pattern a particular person uses in the names he registers.

On most registrars' websites, there is a WhoIs button. You put in a domain name and it tells you all kinds of information, including the registrar the owner used. If you're lucky, the registrar you're searching will only return information on sites registered through them. This makes the search a lot faster.

This will take a lot of time. If anybody else knows of a better method, fire away.