Double Post, but just letting anybody who cares to know that there's a Shiny Tapu Koko being distributed via mystery gift right now, which will last until 8/14. As far as I know there's no other way to get one because legendaries aren't allowed to be shiny via normal encounters.
There's also been a number of Mega Stone distributions via Mystery Gift passwords if you really wanted to use some of the ones missing from Sun/Moon.
M2DESCENT for Mewtwo X/Y
INTIMIDATE for Mawile, Medicham, Audino, Beedrill
AZUL for Pidgeot, Steelix, Heracross, Houndoom
MATSUBUSA for Sceptile, Blaziken, Swampert, Banette, Camerupt
There's still quite a few missing but they'll probably come eventually. Really hope the next time around they just include them in the game, don't really understand the purpose of barring game features like this.