Look up Imax.com and place /theatres at end of the URL to find the closest Imax theater in your city. Know that this is the most popular 3D format for many people, so most of these theaters will be in the bigger cities and likely more crowded.
Consider that because Imax screens aren't quite as plentiful as Real D 3D theaters are, go to reald.com and their theater locator section for info on theaters in your local area. Most large to mid-size cities have at least one Real D-equipped theater.
Go for one of the best alternative theaters in 3D, Dolby 3D, if an Imax or Real D theater is still too far away from your local town. Use Dolby.com and their "find-a-cinema" section, where a reliable list exists of theaters in your area. Understand that many theaters in every town and city choose this format over Imax and Real D for the simple reason of cost in retrofitting.
Go to xpandcinema.com and add /world-map at end of the URL to find theaters using this newer 3D format. Because this format is so new, the world map locator is going to be overly general and chances are only the biggest cities have one. Take the time to investigate XpanD as a new 3D alternative. Many fans say gives the best movie-going experience out of any of the above.