Most reputable site I've seen is:
Escorts, BDSM, Massage and More! - Eros Guide to escorts
Most providers do ask for references from previous providers or that you register on 3rd party sites. Definitely check out the reviews first, there are several sites. Sorry, I don't have the URL's handy, but you can Google them.
Edit: Just remembered one, the Erotic Review:
http://www.theeroticreview.com/