Articuno can be found in Seafoam Islands. You need to know SURF and STRENGTH to be able to go there. Follow these instructions to make it to Seafoam Islands and out to the other side. Remember to: buy extra pokeballs, greatballs and ultraballs (I suggest carrying 30 of each pokeball, greatball and ultraballs), and bring your masterball if you still have it. Make sure you heal your Pokemon; and bring Pokemon of at least level 55. Articuno is at level 50 and it WILL put up quite a fight. The other side of the cave will take you to Cinnabar Island. Good luck!