OT: If you are looking for top of the line graphics then best place to find a designer is HERE. It's all dependant on what you are looking for really...
if you're looking for people that could for you, behance is your best bet
if you're simply looking for amazing artwork, top of the line, you should check out art expo's like depthcore or cosmosys
Finding a professional is like browsing in a candy shop. You have to look around for a flavor you prefer. You could find a professional that does professional work, but doesn't meet your style. Browse through some sites (like Behance; -Le Me- reference), or even do it the old fashion way: Google.
Look for designs that YOU like, above all else. Don't settle with a professional that doesn't have a similar perspective.
Edit:
JEEZ -Le Me-
cosmosys has some incredible work. That's ridiculous.