Players who wish to mega evolve their Pokemon in a battle may not need to scour the region and high seas for mega stones in Pokemon Let’s Go Pikachu/Eevee.
According to a Leaf by dataminer Kurt (@kaphotics), some mega stones may actually be purchasable in game after you have defeated the elite four and champion.
So that should answer a thing or two about mega evolution in Pokemon Let’s Go Pikachu/Eevee
Th-thank you for your purchase. Do you want to buy any other stones?
I…I can’t believe that you bought them all… That’s just like a Champion. I hope you’ll make full use of those mega-awesome stones…
— Kurt (@Kaphotics) November 11, 2018