I normally just walk up to girls in public places and casually speak with them about the possibility of them participating in my amateur modeling shoots. I went though an agency once, but I only shot just a few girls. It is just much safer (and more convenient) to advertise online looking for models. I do this quite a bit too. However, if you take the "finding girls on the street" approach, you must be very professional, as well as respectful in terms of your approach.
If you don't feel comfortable with using such a direct approach in order to recruit female talent, I'd try to hook up with an agency or either create some ads that are local to your area and post them online. Or of course, you can just go out and recruit the models on your own.
Good luck to you!
