A Dusk Stone can be found in Stow-on-Side. It's behind the Pokemon Center there. Take the path around the right side of the Pokemon Center and you'll find a small red pokeball pickup behind the building. It contains the Dusk Stone.
You can find one at the Lake of Outrage or Stow-on-Side. One can also be obtained from the digging duo or from a Pokémon returning from a PokeJob.