Most people will just say "anywhere."
Wherever there are girls, you can meet them.
There is no magic place where everyone finds their perfect match.
You just have to supress your nerves, take a deep breath, and start chatting with the cute girl you're checking out.
I would suggest not doing it as they walk by though
...do it when you and the said girl are near each other, standing still for some reason (looking for books/products in a bookstore/pharmacy/whatever...in an elevator...in a line in a store...)
And a good opener/conversation starter is always good.
But there are lots of posts about this, I think.