The Black Belt, Expert Belt, and Focus Sash are all great items that can be held by your Pokémon. Each one of these items has its own effect. The Black Belt increases the power of fighting-type moves. Meanwhile, Finally, the Expert Belt boosts the power of super-effective moves. and the Focus Sash allows the holder to survive a hit that would normally K.O. them. Here is where you can find all these items in Pokémon Brilliant Diamond and Shining Pearl.
Before you can get any of these items, you will first need to progress far enough in the game to obtain and use the hidden move Surf. Surf is given to you by Cynthia’s grandmother when you reach Celestic Town and talk to her in the ruins. You gain the ability to use it outside of battle after you defeat the Canalave City gym leader.
Once you are able to use Surf outside of battle, travel to Sandgem town and head south to Route 219. From here, use Surf on the water and travel over to Route 221. When you make landfall, travel north to the house in the forest. Head inside to find an old man. He will tell you that he wants to see a Pokémon of a certain level. This level changes each day.
The first time you show him a Pokémon of the appropriate level, he will reward you with the Black Belt. The second time, he will reward you with the Expert Belt. The third time, he will reward you with the Focus Sash.