Many different lists of martial arts schools can be found online. It depends on the martial art, but good places to start are the sites UsaDojo.com and Dojos.info, which each list thousands of different dojos.
In addition to the sites UsaDojo.com and Dojos.info, you can also find lists of martial arts schools in your area in your local phone book. Practitioners of martial arts in your area may also be able to provide you with recommendations and descriptions of the different schools. However, the Internet is by far your best resource when searching for lists of martial arts schools in your area.