Elance is probably your best bet. I'm a ghostwriter, but I can't take on any more projects atm. I know of several ghostwriters who have their own website, and you can go that route, but it tends to be expensive.
Whoever you choose, make sure they are a native English speaker, have a good grasp on grammar and are proficient in researching. It's also a good idea to make it a requirement that they be up to date on the latest AP Style Guide. This will filter out a lot of wannabes.
Make a contract, don't pay by the hour.