Good places to find girls flexing their muscles would be at any athletic or sporting event. Gyms and dance classes are also good options. Some women also compete in events like Ms. Fitness contests.
There are multiple women's bodybuilding events across the United States, according to Natural Body Building Events; girls are likely to flex at these muscle-focused events.
Some other places to find girls flexing their muscles would be track and field events, ballet classes, gymnastics classes and cheerleading events. Many girls take self defense or boxing classes as well, which require them to stretch and flex to warm up. There are even websites like Girl Flex devoted to photographs and video clips of girls flexing muscles like their biceps.