Only available from 14/07/2017 to 14/08/2017 so grab it while you can
To get Tapu Koko in your Pokémon Sun or Pokémon Moon game:
1) Select Mystery Gift on the main menu.
2) Select Receive Gift.
3) Select Get via internet, and then Yes to connect to the internet.
4) Watch as you receive Tapu Koko.
5) Speak to the delivery person in any Pokémon Center to pick up Tapu Koko.
6) Be sure to save your game!