Registrar. You can't buy directly from the registry unless you become registrar yourself.

Some other important tips:

- make sure auto-renew is enabled with a reliable credit card

- make sure to update your personal data once a year. Registrars are mandated by the registry to send a reminder email once a year, but for an important domain I'd also set a recurring reminder every year, 2 weeks before expiration to make sure that everything is ok. You probably shouldn't "setup and *forget*" your domain names

- if you can afford it, make sure to register multiple years at once, the max is usually 10 years, but I'd recommend registering for only 9 years, because if you need to transfer to another registrar for some reason, the registration will be bumped 1 year. If you're already at 10, you're stuck.