I haven't played USUM yet so I'm unsure if Pokémon pelago is still there. If it is though, you can send your Pokémon stone hunting and get a chance to dig up some shiny stones ^^
You can get an infinite number of evolution stones via the Pelago by doing the Path for Brilliant-Stone Hunting or Interesting-Item Hunting, Brilliant-Stone Hunting is specifically for evolution stones although is random what you get you can get all the evolution stones as well as a few none evolution triggering ones too.