There are several registrar companies that sell already existing domain name. I suggest visiting the Namecheap marketplace of existing domain names for several reasons. First of all, you can buy and immediately get a domain name from the Namecheap website. There is no need to negotiate the price. You can filter domain names by price range or category. For your information, you have to be snappy because people order the most interesting domain names quickly. Then, you can take advantage of all Namecheap features like the Whoisguard to hide personal details (depending on the domain TLD).
You can find existing domain names from one of the following category:
Brand/ Name Identity