I've used Dribbble to find UI and UX designers with a ton of success. It's time consuming to message people but there are some ways to speed it up. These are my pro-tips:
1. Always email them rather using the messaging service provided by Dribbble. Works much, much better.
2. Always look at their personal website and/or Twitter account to see if they're *really* available for hire. The "hire me" button on profiles is often misleading.
3. Don't be afraid to hire people outside of the United States or the SF Bay Area. It's cheaper outside of SV and it's just as easy to trade emails with someone in North Carolina as it is with someone in San Jose.
4. Be thorough in your asks. The better your wireframes are, the quicker a designer will be able to work, and the more you'll know exactly what you're asking for. This will make your relationship smoother and the final bill, cheaper.
5. Pay them to test out their skill sets. Have them complete a project for pay that represents the first 5-10% of the work you need done. If you don't like what they do, you can cut them loose for a fraction of the cost. I do this with multiple designers at the start of of a project.