I'm not sure if you know, but do you have ORAS? After you get a Diancie (from trade or something) you go into a Pokémon Center where a short cut scene will play, and you get one free.
You can't get it in X or Y, sadly. Not even if you trade a Pokémon holding it. X and Y aren't coded for Mega Diancie.
Sorry if you already knew this. It's just the quickest way to get it if you already have Diancie!