I chose Delta Charmander as my starter and, for some reason, I turned Purity Mode on midgame (crazy me :[ ), and I picked up the Charizardite Y in Perfection Cult’s base instead of the Delta Charizardite (that time I still didn’t know the mega stone is based on my starter so I thought it was just a random Charizardite sitting there). The game was saved beyond that point so is there still a way to get the Delta Charizardite? Thanks.
nope
Revert to a previous save backup if that is still valid.
Thanks but I guess I’ll stick with my non-mega-able Delta Charizard 
There is an option of requesting a trade. You can try at the Trade section within the forum.
Oh yeah thanks for that 