Dusk Stone

Team Galactic's HQ - Veilstone City: This is your first guaranteed spot, but there's a catch. You'll need to have defeated Gym Leader Candice in Snowpoint City first. After that, head back to Veilstone City and find a Grunt hanging out in front of the Team Galactic HQ. Talk to him, and he'll cough up the key you need to get inside. Once you're in, you'll be able to grab the Dusk Stone.

Victory Road (B1F - after unlocking National Dex): This one's a bit more involved and requires you to have the National Pokédex unlocked. Once you do, fly to the Pokémon League and head back into Victory Road. You'll need to navigate through the area: cross the long bridge, go down, head left, go up, turn left again, and then go up the stairs behind a trainer. After crossing another bridge to the right, you'll enter a cave that was previously blocked by a man. Go through this cave, down some steps, through a foggy room, and up the next set of steps. You'll emerge into a new cave area. Your goal is to head north along the path until you reach some rocks you can move with HM Strength. The Dusk Stone is hidden behind these rocks in the top left corner.

Bogsunk Cavern - Grand Underground: This is a more straightforward find if you're exploring the Grand Underground. Just head to the western region of Bogsunk Cavern, and you should be able to find a Dusk Stone there.