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.
Go towards the south on Fuchsia City until you reach the coast. Surf down then turn to the left to reach Seafoam Island. There will be a cave there, get inside.
When you enter the cave, you can see a boulder to the right and a hole. Make one of your pokemon push the boulder through that hole.
On that same floor, there is another boulder to the other side, so ignore the ladder for now, go to the other side and push that other boulder into the hole once again.
Do not go into the ladder, go back to where the first hole is and drop down through it. Make sure you push the boulders into those holes, they will be necessary to block the current, so that you can surf in the cave's lower level.
The rest is pretty much self explanatory, once you go up the opposite ladder and the second boulder is set you can surf toward the left. You will find a similar puzzle toward the south, find a way to get one boulder into one hole and another in the other hole. If you drop two boulders into the same hole, don't worry just drop down one of the holes and use the ladders to bring you back to the level and try again.
Once the boulders are in the right place, you can drop down one of the holes and either surf up (to the leftmost) to get Articuno. Once you have captured Articuno, get to land and make sure you take the upper right most ladder. This will take you up until you can get to the other side.
The other side of the cave will take you to Cinnabar Island.