You have to find it at the nature preserve. From your previous question it seems like youre at the elite four so you have to beat that first. After you defeat the elite four, you have to see every Pokemon in the new unova pokedex except for event Pokemon(victini, keldeo, meloetta, and genesect). After you do that go to professor juniper in nuvema town and she will give you the permit and then go to mistralton city and take the plane to the nature preserve and you will find it at the center. There will be a stationary one there and it will always be shiny.
Source: http://bulbapedia.bulbagarden.net/wiki/Nature_Preserve
