Buying a domain through WordPress is most direct via WordPress.com , which acts as a built-in registrar. Standard extensions like typically cost between $13 and $18 per year . While the software from WordPress.org is free, it does not sell domains directly; instead, you must purchase one from a third-party registrar and connect it to your hosting. Buying a Domain on WordPress.com
WordPress.com offers an "all-in-one" experience where your domain registration and website hosting are managed in one place. Manage Your Domains | WordPress.com Support