I got a pokemon that needs it to evolve but I don't know where to get one.
You can get one by winning the Bug-Catching Contest, or you can buy one at the Pokeathlon on certain days.
The Dusk stone can be won by winning 1st place in the bug catching contest, althogh you may get a different stone instead. You can get one in Tohjo Falls as well.
You have to trade your Foretrees for a Beldum holding a Dusk stone in-game at Silph Co. in Saffron City.