If you don't get it from seafoam islands you must have a friend trade you one from there leafgreen or firered or get one from Pokemon xd gale of darkness.
seafoam island
You look for Articuno.
Articuno is somewhere in seafoam islands.you can find it in the seafoam islands from fuchisia NOT the island with POKEMON mansion
there are seel and articuno lv 50 come stocked with ultra balls
You go to seafoam island from the right side not the left, you can figure out the rest from there
In Pokémon LeafGreen Articuno can be found in the Seafoam Islands, Zapdos can be found in the Power Plant and Moltres can be found with One Island's Mt. Ember.
zapdos is in the power plant in route 11 moltres is at mt. ember on one island articuno is at icefall cave on four island
Articuno is in Seafoam Island.
Seafoam Island - East of Cinnabar Island - Southwest of Fuschia City
Zapdos is in abandoned power plant Articuno is in Sea foam islands Moltres is in volcano on 1 island Mewtwo is in Cerulean cave
mew is at faraway island. deoxys is at birth island. mewtwo is at cleurean cave. zapdos is at the power plant. articuno is at seafoam islands. moltres is at mt ember.celebi and jirachi i do not know.
no go to fushia city and use surf at the water bellow go into the island articuno will be waiting