Actually, you can buy it at the Stone Emporium.
A specialty shop that sells evolutionary stones. 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. The prices of the Mega Stones begin at 1,000,000, but the man will sell them for as as low as 10,000 depending on the player's stylishness.
So yeah. An easy way to lower the prices by a ton is to buy 99 Premier Balls at the Poke Ball Boutique which maxes out your style.