The best place to find nearly every evolution stone in Pokémon Sw/Sh in at the Lake of Outrage. They respawn periodically, like most other items, so it is really easy to have Pokémon evolving all over the place is you don't mind to wait a little while. All you have to do is grab your trusty bike and use the water-biking thingy (unlocked on Route 9, Galar.) and go on over and ride around the circle of stones, picking up the stones along the way.
Sources: My own personal experience (with this I now have:
•Four Sun Stones
•Five Fire Stones
•Two Thunder Stones
•Four Water Stones
•Two Leaf Stones
•Three Shiny Stones
•Three Dusk Stones
•Three Dawn Stones
•Three Ice Stones
It's an unbelievably easy mechanic to get all the Evolution stones you need!