I want it.
Firsly did you choose Chespin as your starter? If so you'll find Articuno randomly. If you've seen it once you can check its location in the pokedex.
Chespin - Articuno
Fennekin - Zapdos
Froakie - Moltres
Hope I Helped.
Articuno wanders the routes IF you chose Chespin. he flys to random routes and automatically flees. But after 12 encounters, he will be in the Sea Spirits Den, where you can fight, and catch him.
Source; Experience(I caught him)