I know some people who have had success with odesk.com and elance.com. It looks like a bidding war on those sites, though, so you have to build some reputation before you can command higher rates.

For my money, the best people to talk to about freelance work are design firms, boutique consulting companies, and recruiters. They always seems to have some expertise they're missing and willing to subcontract out.

Make some calls to firms you trust or have heard of and ask. It may be harder in terms of effort, but I'd bet you'll land a better gig that way.