Before searching for a vocal coach, decide what you need one for. If you are a singer, know the type of music you perform. An operatic vocal coach will teach you different technique than a pop music vocal coach, so if you aren't singing opera, immediately rule out an operatic vocal coach.
Contact local colleges for names of faculty who teach voice. They are the best place to start, as the network is small, and they can often refer you to reputable coaches who can work with your needs. Or, they may even offer to teach you themselves.
Contact music-focused universities, even if they aren't local. Larger networks reach far and wide, and often the faculty at Juilliard, for instance, will know someone in your area they can recommend.
Look through entertainment trade magazines. Backstage Magazine, for example, lists vocal coaches in their "yellow pages," as does Music Connection.
Do an Internet search, but be careful with this. Anyone can say they are a vocal coach, so always ask for references before signing on with anyone.
Check with local theatre groups. Often the musical director is a vocal coach, or can direct you to one.
Ask for recommendations at music stores (by this, I mean retail places that sell musical instruments, not recorded music).
Check music networking sites, and again, ask for recommendations from people who've been taught by these coaches.