We also use
http://www.xxxoutsourcing.com/
Their cartoonists are really the most talented I've seen in the industry. We have one cartoonist that has become so popular his work is regularly featured in AVN Online and other publications. Greg is great to work with and will really help you find the right person to fit your needs. As long as you are willing to put in the time to delegate work to them and make sure they know what you want, they'll do great work for you.