If you have the budget to pay a competitive rate:
Fulltime
- http://www.angel.co - A great platform for startup hiring. you can find founder level talent here.
Freelance
- http://www.dribbble.com - Cream of the crop. For a reasonable fee you can search by location and availability - http://www.behance.net - another great collection of portfolios
If you're not prepared to pay a competitive rate:
Fulltime
- http://www.craigslist.org - I've hired great designers via craigslist who were just starting out, but didn't have much of a portfolio put together.
- Local art schools or universities with design programs - same theory, you'll save by hiring designers at the beginning of their careers
Freelance
Not the best, but definitely the cheapest.
Good luck!
/thread.