If you are talking about the keystone, you can't reach the battle tree without going thru the poni plains. A couple of tourist npcs give them to you after you beat them. Without the keystone, you will not be able to mega evolve any Pokemon
As such, I will refer to the mega stone and to answer your question, no you cannot. The only way to get Charizard's mega stone is after you have beaten Red. The kanto starter mega stones are also not in the battle trees's shop
The only other way for you to get Zard's mega stones before the battle tree is if you find a kind soul to trade you one
