Four more Mega Stones can be downloaded for players of Pokémon Sun and Moon right now. These are used to activate Mega Evolutions for Gardevoir, Gallade, Lopunny and Diancie. Use the code of POYONG from the Mystery Gift option to download these items now.
There is no end date yet known for the distribution, so be sure to get them before it expires!
Thanks to Aurora for the tip.