There are some (not many) UI designers on HN, like me for example.

/shameless plug

I would stay away from Elance type sites unless cost is your only factor. 37signals job board will attract the exact opposite candidates as Elance (read: expensive/overpaid)

Ever think of hiring local?

Attributes to look for would be experience not only in creating the 'web 2.0 look' but also solid experience in eccomerce and application development. Too many designers only know how to make something look good, and fail to consider that you're doing business on the web. Avoid designers who rely on wiz-bang features, even a boring business like portfolio is better than one filled with tons of Flash and AJAX.