Well there are two stones to get, the keystone used by the trainer, which can resonate with any mega stone, which is specific to Pokemon.

I think it'd be better to make it that the first mega stone you get, is almost essentially given to you, but you get your choice on it, but ones after that, you have to go find.

As for the keystone, you can even start with it maybe? Since it's useless without the mega stone, plus to achieve mega evolution, your trainer needs an intense bond with the Pokemon too which is stated in descriptions of mega evolution (even though in the game it is for obvious reasons, pretty much instant).

I think most people will only want one anyway for the lynchpin in their team.