If you have an event Celebi, you can get a Zorua with it. Put Celebi in your party and go to the building in Castelia with a boy and girl talking. Talk to the girl, then she will ask you to talk to the boy. You will send out Celebi, and the boy will turn into Zorua. Then you can add Zorua to your party if you have a Pokeball.
Hope I helped! Source: Experience.