There are 4 ways to get Dusk Stones in Pokemon Platinum:
In Wayward Cave via the secret entrance underneath Cycling Road, you'll find a Dusk Stone hidden on a rock. It's not visible in the overworld but if you interact with the rock you'll get the item.
In the Galactic Warehouse in Veilstone City there's a Dusk Stone as an item on the ground behind the security door when you enter.
In Victory Road there's a Dusk Stone inside the extra chamber that you gain access to after you obtain the National 'Dex.
And multiple Dusk Stones can be obtained as Pickup items. The best odds are a 10% chance of getting a Dusk Stone with any Pickup Pokemon leveled 51+
Many of these locations aren't obvious so they're hard to explain with text-only. But this tutorial page has screenshots that show you exactly how to reach each Dusk Stone: https://guidestrats.com/pokemon-platinum-dusk-stones/
