Otherwise, if you want to manage a domain with DomTool, you must first buy a domain from a domain registrar. Search for "domain registrar" via your search engine of choice to see several of the available options.
HCoop as a whole does not endorse any particular domain registrar. That said, here are a few that some of our members like.
FrankBynum and RobinTempleton recommend https://gandi.net/. They are a French registrar who has a nice English-language site and are one of the only few registrars that declares in the agreement that you actually own the domain name. HCoop uses Gandi for its own domain names.