1. Consider exchanging services with a designer. Costs some development time, but might be worth it for good design work.
2. Ask a designer friend for a favor. You don't want to impose too much, but you might be able to get a discount off the normal design rate.
3. Depending on the type of project, you may be able to find prebuilt templates/themes that you could adapt. Themeforest is a good place to look.
4. Consider using a grid-based CSS framework like Blueprint. It already has all the basic building blocks in place. It won't be beautiful, but should be at least reasonably usable.
If it's something small with benefits to social causes, I know many designers (myself included) would be willing to do it for free or at least, take a loss on it.
Otherwise, you can look on Craigslist/elance or post on designer sites like QBN.