Sedo is where I bought my domain - over Escrow - and is a global marketplace to buy, sell, and park domain names. They've over 18 million domain names for sale, but the big part is that they're a huge, well-respected company. This may not be important if you're spending $150 on a domain, but in a $15,000+ purchase - which happens in many cases - you will be potentially buying from a foreign entity, that you don't know, and many of them will suggest an escrow payment. This means you put the money in an account owned by a third party that will hold it and only pass it on once the domain has been transferred over into their name (which in this case is what Sedo does). This can be quite disconcerting - but Sedo is well-known and has done many, many deals - and the process is about as full of hand-holding as handing tens or hundreds of thousands of dollars for a domain name can be.
If you have a web site, you should seriously consider registering your own domain name. A domain name is a name like "thesitewizard.com" or "thefreecountry.com", which you can use to refer to your website. Note that you do not have to be a company or organisation ("organization" if you use a different variant of English) to register a domain name. Any individual can do it too.
Domain name registrars, in case you didn't know, are the companies that manage all the website domain names you'll see (EG: Inc.com) - the fingerprint of your business, which may also include the hosting and privacy measures. For example, by default many companies will actually over-price the simple process of keeping your registration information private (which if it isn't can be found via a simple Whois search).
There are tons of great domain names available for registration and it can take as little as five minutes to register one at any domain name retailer. These online stores make it very easy for you to search for available domain names with name suggestion tools. They also offer complementary services, such as Web hosting and website design so you can register your desired domain name and set up a website all at once.