If it has been awhile since you visited the Alola region, you will want to check it out again soon. Starting in October you can receive a shiny legendary Pokémon for Sun, Ultra Sun, Moon or Ultra Moon. But you are going to need the Pokémon Pass app first.
From October 21 through November 10 visit any participating GameStop location. Use the app and receive a code to use in your game. A shiny level 60 Lunala will be in Sun or Ultra Sun having Moongeist Beam, Psyshock, Moonblast and Moonlight. If you have Moon or Ultra Moon, it will feature shiny Solgaleo with Sunsteel Strike, Zen Headbutt, Noble Roar and Morning Sun.