I'm not sure of any service for hiring an SEO beyond the standard marketplaces like oDesk & Freelancer.com. I've worked with a handful of "SEO experts" over the last few years and can tell you that it's important to be careful. I've wasted thousands of dollars bringing in "experts" when things get busy and the results have been everywhere from horrid to exceptional.

For me, the best approach is to take the time to scout out talent as if you're scouting for a professional sports team. Refuse to take the easy approach by putting an ad on oDesk and hoping that the right people respond. Instead, visit communities like Inbound.org, GrowthHackers.com, Moz.com or even Clarity to begin your hunt!

The community on Moz.com is likely going to provide you with the easiest route to finding talent. You could upload a post in their community section asking for talent and likely have a decent response. On the flipside, you could also do the heavy lifting on your own by browsing comments and other threads to find people who responded with insightful & high quality information.

I've hired SEO's and marketers everywhere from Europe to Canada - Happy to help out with resourcing or helping find the right talent if you ever want to chat.