Zorua is a special pokemon in Pokemon Black that can’t be caught using regular methods.
You can only get Zorua if you have an event Celebi in your party. Take your event Celebi to Celestia City near the gates. Look for a group of two people talking, and try to talk to one of them. They will turn into Zorua, and you will be able to engage and catch it.
Zorua can be bred however, so there should be plenty of people on WiFi willing to trade them.