Keep in mind that you re only see the really good photos they ve taken. What you should look for is a consistency of good photos in their book. Once you ve decided that you like a particular photographer you need to meet with them. The quality of your headshot depends almost entirely on how your personalities mix. Do you feel like they re really interested in working with you , or are you just one of many clients.
Most photographers advertise in Backstage.
Another place to look is in the lobby of Taranto Labs in New York City. They have on display the portfolios of some of the best photographers in New York. A commercial shot is usually one where the actor is smiling, open, friendly, and glowing with enthusiasm. The legit shot is more focused, intense, and dramatic. Such photos are used for plays, film, and television.