Ninja athletes aren`t just found on TV. NBC`s "American Ninja Warrior," which has competitors racing through ingenious and diabolical obstacles, has sparked the development of ninja gyms and leagues across the country. The locales train future ninja warriors, and often cater to kids, who come for parties and training. But they also welcome drop-in visitors of all ages. [It`s a place for people who watch the show and say `I can do that` to see if they really can," says Grant McCartney, a top competitor who promotes the sport around the world. He shares some favorite gyms.