A final serial code has been revealed on the official Japanese Pokémon website for the last Mega Stones not previously release in the seventh generation games. You can download the Mega Stones for Latias, Latios, Ampharos and Altaria with the serial code of DRACHE. Enter the code through the Mystery Gift option via the main menu.
You have until the 31st of October to download all the Mega Stones. The serial codes are as follows:
Be sure to download them before it’s too late!
Thanks to HeroLinik and colours for the tip.