Oh man, conventions are a big yes. Haha when I used to do cons in high school/undergrad my girl friends all met guys at cons... and the guys got a ton of action too, especially if they cosplayed popular characters.
I, being a nerdy girl, would say the best place to find a nerdy girl would not be the library or math class or whatever else was said. Because during those times I, and other nerd girls, are otherwise occupied with finding a book that hasn't been read in the library or trying to learn in math. You have to approach a girl when she isn't busy doing something, or you just come across as annoying, and that's not a good first impression. A guy would get a big plus from me by randomly sitting with my lonesome self at lunch and starting a nerdy conversation with me i.e. what stuff we have in common. Another good place is Barnes and Noble or on a nerdy website, such as this. Generally people on this website aren't creepy people and are genuinely nice nerdy people.
anywhere and everywhere if you look closely enough you will see that there are nerdy girls seeping out of the woodwork as long as you look in the right places. Club and cool places are total out of the question. nerds are in those place posers.
Look in bookshops, libaries. any fast food resturants usally out of the way unoticed by the overwelling population of guys.
Of course what I say is complete guess work as I go to a all girls school so have no expericence with guys. I also live in england and the bookshops I go to are nearly empty and the libary is small lonely and has a usless teen age section.
One thing I do know is the hard core nerd girls will be in none of these places if they are like be.
I spend my life reading and watching TV at home. Order most my books online unless I'm out of ideas for books I want. and in general being total unsociable