Dribbble.com is fantastic, clicking on the user to find their personal website makes it approachable.
There is also Forrst.com, though the quality has been slipping over the past year but they have a directory of designers.
Don't forget builditwith.me who make it incredibly easy for you to search for designers specifically.
If it's a startup and the budget is small then finding a like minded individual is your best bet, who is willing to help out or potentially swap services, rather than approaching a freelance designer directly.