Sometimes there are clubs or interest groups, such as a Linux users group. These are generally open to anyone who is interested and while they are heavily male in their membership, that is one place nerd girls can be found.
If there is a college or university near you consider taking some classes through the computer department,. particularly classes in game design. You should find some nerds of the fairer sex there.
Hang out in the computer section of someplace like Barnes & Noble and see who picks up what books. Then find a reason to strike up a conversation.
Head to the nearest Internet cafe and hang out, watching for girls who play online games.