If you want a subdomain of some existing domain, you should find the contact point for the parent domain rather than asking one of the above top-level registrars. There should be a convention that registrar@domain or hostmaster@domain for any given domain will always be an alias for that domain's registrar (somewhat analogous to postmaster), but there is no such convention. Try it as a last resort, but first you should examine the SOA record for the domain and send mail to the ``responsible person'' shown therein. You can also try whois.