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.
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:
Main Game (before Elite Four)
- Charizardite X/Y - 1,000,000
- Venusaurite - 1,000,000
- Blastoisenite - 1,000,000
Post Game (after Elite Four)
- Charizardite X/Y - 150,000
- Venusaurite - 150,000
- Blastoisenite - 150,000
According to the Pokemon Guide, pointed out by BenBrocka, the price can get significantly lower based on how "stylish" you become:
I have not beaten the Elite Four, but I have bought the Stone for Charizard (I picked Bulbasaur) for a mere 10.000. I can confirm that Lumiose Style is the only relevant factor.
The original information provided in the previous answers probably comes from a source that checked only after doing several style-raising actions and beating the Elite Four without checking in-between.
For ways to raise your Style, check the picture in Retrosaurs answer.
One kind of lame way I have tested and can confirm is buying many Premier Balls one by one. Exiting the store or the buy dialog is not necessary, you can just select the Premier Ball and mash A. In my testing, roughly 120 Balls were enough, I've seen reports of less and have done Style-raising actions before that. My assumption is that with 0 Style, 150 should do.
I'm not sure what determines which Stone you get from him, but he only ever offered me Charizardite. While I didn't pay attention to the days and it might actually be related to when you talk to him, I think the third Stone might well be unlocked after beating the Elite Four, possibly only after upgrading your Mega Ring.
You can buy it in Luminose city at the stone emporium for 150,000 after the elite four. Prior to that, they're 1,000,000.
