Sorry to say, but the other answer is wrong. You can only get Charizadite Y in Pokemon Y. The man in the Stone Emporium only sells Blastoisinites and Venusaurites. Here's my source
A man in the store will offer to sell the player the remaining Mega Stones for the two Kanto starter Pokémon they did not receive from Professor Sycamore.. After entering the Hall of Fame, the owner will first offer the Mega Stone for the Kanto starter that is super-effective to the one chosen by the player.
So, the only way to get a Charizadite Y in X is to trade. Here's extra confirmation of my statement.
Hope I helped :)