If you don't have a tight budget, I would suggest: airbagindustries.com, happycog.com, blueflavor.com or 31three.com - they are all well known designers / design studios and do great work.

If you're looking for "cheap" work, you could visit FreelanceSwitch.com and post on their job board or get involved in their forum and try to find someone that way. Also SitePoint.com allows you to hold contests to pick the best work, though most good graphic designers stay away from the contests because it is considered unprofessional to submit work on spec.