The Black Belt, Expert Belt, and Focus Belt are all great items that your Pokémon can hold. Each of these elements has its own effect. The Black Belt increases the power of combat-type moves, the Expert Belt increases the power of super-effective moves, and the Focus Sash allows the holder to survive a blow that would normally KO them. This is where you can find all of these items in Pokémon Brilliant Diamond and Brilliant Pearl.
Before you can get one of these items, you'll first need to advance far enough in the game to get and use the hidden Surf move. Surfing is given to you by Cynthia's grandmother when she reaches Celestic Town and talks to her in the ruins. Gain the ability to use it outside of battle after defeating the Canalave City Gym Leader.
Once you are able to surf outside of battle, go to the town of Sandgem and head south to Route 219. From there, use Surf on the water and travel to Route 221. When you land, travel north. up to the house in the forest. Enter to find an old man. He will tell you that he wants to see a Pokémon of a certain level. This level changes every day.
The first time you show him a Pokémon of the appropriate level, he will reward you with the Black Belt. The second time around, he'll reward you with the expert's belt. The third time, he will reward you with the Focus Sash.