There are many ways to have someone "throw some testing" at your site. Which way is best for you depends on
- Your needs
- Your ability to provide input to the process
- Your timeframe
- Your budget
Some folks just post something on a forum where professional testers hang out (like here, or sqaforums.com) and ask for people to "test" their site. The good: You get professional testers looking at your site. The bad: You aren't likely to get any kind of deep testing without offering some sort of compensation.
Some folks ask friendly bloggers (like me) to post a link to their site, sometimes with a page explaining how to contact the site owner for further instructions/information. The good: Inexpensive. The bad: You have to do the work of weeding out the non-useful from the useful folks.
Some folks use a Beta (open or closed) as a way to gain insight into the functionality and quality of their site. The good: It can be either free or very inexpensive. If you already have a wide audience, it can be quick to get many "testers". The bad: You will likely get casual users, rather than professional testers.
Some folks use less-expensive organizations like uTest or others to provide testing services. The good: Relatively inexpensive. Some decent infrastructure already in place to organize and manage the testing process. The bad: A bunch of faceless testers with a wide range of abilities. If you don't provide solid input, you likely won't get much back.
Some folks recruit their professional tester friends. The good: Your friends want to help you. The bad: You are imposing on their time.
Many folks go and hire a professional tester for a short-duration project. The good: You get real, professional testing. The bad: More expensive, requires real work on your part.
So you have to ask yourself - How serious is my need? What's it worth to me (in terms of my money and my time)?