Our site has a growing wiki that is based on wikimedia. I'm looking for people with experience designing the interface, custom skin, etc How do I go to find one?
First, the software is caled Mediawiki (not Wikimedia, which is the name of the foundation) - this could be the route of some of your difficulties?
Second, I'd recommend searching "Mediawiki Themes" and "Mediawiki Templates" and contact the designers from the sites you find. Here are a few places with designs/themes, that may be open to doing something custom for you (courtesy of Google):
Best of luck!
You might find one on a freelancer/contest marketplace:
Can't recommend one in particular though as I haven't used any of them.