To purchase a domain without hosting, you can go for any domain registration.
If you need a stable hosting, then you can refer to ICCAN accredited registrars. To become a domain registrars, the company need to fulfill lot of requirement. That is why, most of the popular hosting provider also a domain registrar.
You can refer to Icann http://www.icann.org/en/registrars/accredited-list.html