There is the "Help Wanted Ads" link on the navigation bar to the left. I don't know how many people look through those though. I only remember it exists when somebody asks if there is a place to request help.
The best thing to do is post right here, and give a little info about the project so that people can gauge whether or not they are interested. It helps if you demonstrate that we won't be wasting our time. Show that you know what you are doing. If people see that you've already made some good progress on the project, or that you've completed past projects, or that you have well articulated, attainable goals, they should be more eager to help.