Got Web / UX Design Skills? Lets Talk.
blog.scoreyard.com
blog.scoreyard.com
Most good designers won't work on spec, but if you look at print designers interested in jumping to web, you could try providing comparable hours of development for their freelance/personal stuff. I see you're interested in trade/barter, luckily it's your best bet.
Also, it sounds like you need a designer who can do front-end, not just any designer. If one of the dev's can pick that up, you've got a wider array of options. While it's becoming more common for designers to know the mechanics of what they're designing for, the good designers who can do front-end are not going to want to work for free.
Good luck!
If you can't afford a designer, then launch the product with the design you have, which is not terrible.