Darn it Jim, I'm a developer, not a writer! Great copy is paramount to search engine rankings and making visitors feel good about whatever the website is offering. What is the best way to go about finding an awesome writer who specializes in writing great copy specifically for the web?
I would strongly encourage you to follow the advice in Derek Sivers' article How to hire a programmer to make your ideas happen. Most of his advice can be applied to hiring a freelance copywriter. The most important part (IMO) is to start with a small part of the project and hire multiple writers, then choose the best among those to finish the project.
Elance.com is a possibility, they have professional writers that you can hire by the job.
We used Elance.com as George mentioned, CraigsList and ODesk are also options. If you have a college nearby, see if you can pick up an intern or two. AfterCollege is a site for posting that type of job. Dice, Monster, or your local paper.
The main thing we learned was to get several and do small test projects first.