Mega Stone of chosen Starter Pokemon buyable?

Not for Charizard, at the Stone Emporium in Lumiose City you can buy for 1,000,000 before and (as low as) 10,000 two other Mega Stones after you beat the Elite Four at the end of the story.

I bought Blastonite and Venusaurite. I can't only buy other Chairzardite Y or X it depends from your Pokemon version.

I was able to bring the price down to 10,000 before even having all the gym badges. You have to increase your style. Best way to do so is to go into the poke ball store and buy 100 premiere balls (the 200 ones) but buy them individually...each individual purchase in the town increases your style. I bought 100 of those balls seperately, did all the jobs in the hotel, did a couple of the battles in the resturaunts, and a couple water/fire stones from the stone store. That brought down the price to 10,000 with only my 5 gym badges. If he doesn't drop down to 10,000 go back and buy 10 more balls and keep going back and forth until it drops

Each of the Mega Stones is available exactly once per save file.
You receive the Stone for the Starter you pick from Prof. Sycamore and won't be able to get a second one. The Stones for the other 2 Starters can be bought from the guy left of the counter in Lumiose's Stone Emporium store (details in a related question), but he will only sell you one of each (I believe actually only one prior to beating the Elite Four and the last one after that, but it might be related to the day you talk to him).

The Charizardite exclusive to the Version you're not playing is not obtainable at all.

So to sum it up, assuming you are playing X, you can receive 1 Venusaurite, 1 Blastoisite, 1 Charizardite X and 0 Charizardite Y. If you're playing Y, the Charizardites are reversed.

The only way to get additional ones is to (1) trade for them online or (2) trade them to a friend, erase the save file, start a new adventure and get back the Stones you sent to your friend.

By the way, this applies to all other Mega Stones as well. There is no way to obtain more than one of each Mega Stone without playing through the game twice or trading for it.