This is how you get the starter Mega-Stones. Let's say that you chose Charmander and got Charizardnite. If you go to Vernal Avenue Stone Emporium. There you can buy Venusaurite and Blastoisenite. They can be sold for a lower price, but then you need to get stylish (Won't write on how to get stylish.).
I'll be lazy and give you this link, this link will show you how to get the rest of the Mega-Stones-