I've got to the part of the game where you can choose one of the Kanto region starters.
I'd like to train a Squirtle this time around but I also would really like to be able to see Charizard Y at some point.
I have a Charizard I can transfer up from Pokémon White but I need to know if the megastone for Charizard is something you can get even if you didn't pick Charmander from the Professor.
Best Answer
In Lumiose City there is a "Stone Emporium" that sells...stones.
If you go in, there's usually only a clerk that sells regular stones (Fire, Leaf, Sun, etc.). However, to the left of the clerk there is a man who does sell the Mega Evolution stones.
Note, his prices are tremendously expensive, if you haven't defeated the Elite Four yet. Defeating the Elite Four and the Champion should move the game into postgame, which drastically cuts down the prices for the stones.
Here are the stones he sells:
According to the Pokemon Guide, pointed out by BenBrocka, the price can get significantly lower based on how "stylish" you become: