Articuno can be found in the Seafoam Islands in the Kanto region.
Articuno can only be fought after you defeat the Elite 4. If you encounter him before then he will fly away, but don’t worry he comes back.
If you accidentally defeat Articuno, he will respawn in 7 days. Once you capture Articuno, he will no longer respawn.
You can also receive an Articuno Egg as an Ultra Rare drop from defeating the Legendary Birds Boss at Mt. Ember Summit on One Island.