You can find high end designers on both Odesk and Elance but those ones you need to headhunt yourself. They generally have so much work that they do not need to apply for any jobs - they get headhunted by clients for opportunities. And they are picky so make sure your opportunity sounds appealing and terms are clear.

You could also try specialist marketplace for designers, there are many out there. Just do a search on Google.

Locally, go to a networking event and you'll find many, or join a Meet Up-group for designers.

You could also search local designers on LinkedIn - you can do so for free!

Finding top designers take time and effort. If you do not have the time, consider hiring a recruiter that does the work for you. You can find numerous both on Odesk and Elance.

From my experience, many start-up companies find new clients and win contracts via various freelancing platforms; these are not just for individual contractors. I've worked with companies helping them get started on freelancing websites and win clients. It takes time and effort to start, but once you have established reputation you get invited for jobs and re-hired. When using freelancing platforms your payment is secured which is important for start-up - not all sites offer this though, always read the small print carefully.
You should also network locally. Find an event that is geared towards start-ups and surely someone there needs design work done.
If you have any further questions, just give a call.