Registrars are the organizations or individuals that are allowed access to the automated system of .SX Registry in order to register domain names for their customers or themselves. SX domain names can only be registered via Registrars that are accredited with the Registry.
A full list of Registrars where .SX domain names can be registered is viewable here.?
If you are interested in becoming a Registrar you can review the full procedure, explained here.