Dusk Balls are essential for anyone catching Pokémon at night or in caves, and you’ll thankfully be able to buy them towards the latter half of Pokémon Brilliant Diamond and Shining Pearl.
In order to buy Dusk Balls, you should find yourself in Celestic Town, which is northwest of Solaceon Town and east of Eterna City. To get to this area, however, you’ll have to do a few extra steps. First, you’ll need to scatter the Psyduck blockade by getting Cynthia’s potion and then you’ll need the Defog hidden move for your Pokétch. After that, you’ll be able to go to Route 210 and find your way to Celestic Town.
Once you’ve arrived, go to the top left corner of the town. In this part of the neighborhood, you’ll see a normal grey and brown house beside a green-haired woman and a small pond. There isn’t a Pokémart in town and there are no signs to establish this is a shop, but believe us, they’re selling Dusk Balls in there.
Enter inside and talk to the elderly cashier on the left. He will offer you a bunch of specialty Pokéballs for 1,000 Pokédollars: the Dusk Ball, the Quick Ball, and the Timer Ball.
Once you’ve received the Dusk Ball, proceed to use it in caves and at night where it is more effective. This object can help you catch rock Pokémon and perhaps a legendary Pokémon if you come across one inside a cave. Master Balls, Ultra Balls, and maybe even the Timer Balls might be more effective, but it would be good to keep a few Dusk Balls in your back pocket whenever a legendary Pokémon shows up.