When looking for a high quality UX designer please note that a UX designer is not necessarily a UI designer and vice versa. Some designer are both, like myself, or they might specialize. Dribble has awesome designers but don't always show the UX process that went into the design. Try looking at UX specific sites such as http://uxmag.com/ or LinkedIn groups.
Be sure to ask UX designers about their process and samples, such as user stories, case flows, wireframes, architecture etc. You need more than just a great design, it needs to be structured to meet your business goals and show results.

For example, a client that had a great looking site asked me to review the site UX and find out why their user retention and sales were low. After evaluating the site I discovered that the shopping cart was a one way path and customers had no way to purchase other items or return to the site. Just adding incentives and a clear return path greatly increased their sales.

I have been hired to define the UX and architecture for a site or app, and then pass on, or collaborate with UI designer. I have also just created the UI, working with a client's UX team. But i also do all parts of the process depending on the project. Hope this helps!
Please contact me if you have any other questions.
Duncan