If you entered and participated in the Weakness Cup that took place back in July, you can claim your prize! Four Mega Stones are available to all those who entered and met the battle limit for the competition.
The Ampharosite, Altarianite, Latiosite, and Latiasite Mega Stones can be claimed by signing in into PGL and obtaining a special serial code for your game. These Mega Stones will be available for download up until August 30, 2017, at 23:59 UTC.
Surely enough these will be available to everyone later this year via a serial code.