I always start at Sportsbook Review dot com. There are several sites that claim to be industry watchdogs for online sportsbooks.

Read their reviews and ratings of sportsbooks. They have hundreds of sportsbooks listed. Read other people's complaints and resolution, if any. There are more sportsbook scams than casino scams, so use due diligence.
http://www.gowinsportsbooks.com
http://www.gowinforum.com