On Akala Island, behind the Fossil Restoration Park's RV, there is a Thunder Stone. In Po Town on Ula'Ula island, if you go inside the Team Skull HQ, and go through the doors on the first door on the right, there will be a pool with an Ice Stone in it. You can also buy the fire, water, electric, and leaf stones from Olivia's Shop for 3000 Pokedollars. Most of the stones/rocks can be found through Poke Pelago.
Source- Experience
I forgot the rest, but this article tells you what Pokemon use the evolutionary stone and where to find them.
