I'm 22 and I don't drink. I find the taste of alcohol completely repulsive. Girls (and guys) who don't drink are not extinct; I know plenty of people my age who don't drink. Maybe we subconsciously gravitate towards each other, I don't know. You probably won't find us in bars or clubs...but as far as finding non-drinking people, that is something that you'll have to take the time to find out.
I agree with Littletad. Even if you meet someone at a library, they could be a drinker at night. School activites, volunteer places, at work...can be places to find someone. Do something you like to do, and there might be someone there of interest. For example, I love snowboarding, and I have met really nice guys on the slopes (that don't drink).
at your age its going to be very hard because even most of the so-called "nice girls" mostly go through a phase in their late teens/early 20s when they do a lot of partying and drinking. most grow out of it by the time theyre 25 though, except occasionally and socially. the best thing I can tell you is to join a group or club of people who share a similar hobby with you. you'll no doubt meet tons of nice girls there and you know they share a passion with you, which is a plus.
There's plenty of girls who don't drink. I would suggest meeting girls at a coffee shop, clubs, and activities at school. There's no real easy way to find a specific non-drinker though. Every girl is different, and the only way you'll learn about their drinking habits is to ask them.