My company does UI/Design and usability testing for developers (as well as development for designers/ad agencies).
Although I tend to work more on the development side of things I can tell you that finding competent, knowledgeable designers who understand how to create usable interfaces that will help increase conversion rates and still look appealing to the target market is no easy task.
We interviewed about 100 designers before we found a few that understood that design needs to support business objectives and solve business problems (like how do we get more sign-ups directly from our home page?)
Many of the designers out there are simply focused on making websites/interfaces that look good but don't actually support business objectives or customer needs.
The best designers take a problem solving approach instead of a purely aesthetic one. So when interviewing them be sure to ask plenty of "problem solving" type questions. This will help you weed out the "artsy" types from the serious designers.
One more piece of advice: stay away from sites like 99designs etc since the only designers on there seem to be ones who are just starting out and have very little experience solving real world problems. 99Designs may be great for small mom and pop outfits (think local deli, florist, etc) but it doesn't work well for serious start ups that will need to launch with an interface optimized for conversions.
If you are looking for a rock solid design team that also understands the development process, give me a shout. We're currently wrapping up some work for another incubator.