[REQUEST] (question actually) On Mega Stones not in Pokemon X

[thecofybean]

I only have access to Pokemon X as of this moment.
And I am trully a newbie (like 1.5 months old?).

Anyway my question is, how can I ever have my own mega stones not found in Pokemon X, most specifically,

Heracronite
Galladite
Charizardite Y

etc etc.

Thanks in advance!