It gets the information from Fandango.com. Input your zipcode there, and see what theaters it shows you. I think it gives something like a 20 mile radius.
Also, you can only buy tickets from theaters that Fandango has contracts with, but they still provide showtime information for other theaters in your area.
For example: AMC tickets are sold by MovieTickets.com, but Fandango will still list showtimes for AMC theaters.