Dusk Balls are essential for anyone who catches Pokémon at night or in caves, and luckily, you'll be able to purchase them around the second half of Pokémon Brilliant Diamond and Brilliant Pearl.
To purchase Dusk Balls, you should be in Celestic Town, which is northwest of Solaceon Town and east of Eevopolis. To get to this area, however, you will have to take a few more steps. First, you'll need to disperse the Psyduck block by getting Cynthia's potion and then you'll need the hidden Defog move for your Pokétch. After that, you can go to Route 210 and find your way to Celestic Town.
Once you arrive, go to the upper left corner of the city. In this part of the neighborhood you will see a normal gray and brown house next to a green-haired woman and a small pond. There is no Pokémart in town and there are no signs to indicate that it is a shop, but trust us, they sell Dusk Balls in there.
Go inside and talk to the elderly cashier on the left. He will offer you a lot of special Pokéballs for 1.000 Pokédollars: the Dusk Ball, the Quick Ball and the Timer Ball.
Once you receive the Dusk Ball, use it in caves and at night where it is most effective. This item can help you catch Rock Pokémon and possibly a Legendary Pokémon if you encounter one inside a cave. Master Balls, Ultra Balls, and maybe even Timer Balls might be more effective, but it would be good to keep some Dusk Balls in your back pocket whenever a Legendary Pokémon shows up.