I am looking for a talented web designer to contribute to an new open source project. Can anybody recommend a good way to find one.
I'm with John Gruber--finding good designers to work for free can be tough. That being said, you might start sifting through some articles on A List Apart. There are some amazing designers who have contributed articles, and if you see one whose aesthetic you like, you could attempt to establish contact. In all seriousness, best of luck to you. I'm glad someone is concerned with his website's interface instead of dwelling only on the so-called hard part.
This is a long shot but you might try forums related to open source graphics software, like GIMP. Any designer using such products might be inclined to contribute to an open source project.
You may want to look for a user group in your area to meet local Web Designers. Here in phoenix there are many usergroups, not only for web developers but also ones for Designers. Of course this may not be an option if you not in a big city, but if you are you can meet a lot of talented folks.
I think there are two kinds of people who might agree to something like this:
Students and beginners who want to pad their portfolios. The best way, IMHO, is to get in touch with some design colleges (the graphic/web/interaction design departments). When I was in college, the administration used to send job offers, including volunteer work. I'm pretty sure that something like this exists in other colleges as well.
Designers who care about open source, and, more importantly, about your specific project. Hype your project like hell, and put up a temporary ugly site with a huge "WE NEED A WEB DESIGNER" banner.