You can only obtain Expert Belts from the elderly man in a house on Route 221. Once per day he will ask you to show him a Pokémon of a random level, and if you do then he’ll reward you with one of three items, one of which is the Expert Belt.
This is a repeatable technique, so you can get multiple Expert Belts this way. But there are no other ways to acquire Expert Belts in Platinum.
As for the item itself:
The Expert Belt is a Held Item that boosts the power of super-effective attacks by 20%.
Expert Belt Location (Step-by-Step)
You can get Expert Belts from the elderly man inside the house on Route 221, just west of Pal Park.
Once per day the man will ask if you can show him a Pokémon of a random level; if you have a Pokémon of this level in your party, the man will award you an item.
The quickest way to reach the elderly man’s house is by Flying to Pal Park and heading west on the path below the stairs.
Head west down the long linear path, passing the Berry patch to your south.
Then turn north as soon as you can when there’s a split in the trees.
And when you turn to go north, make sure you take the right-hand path through the grass.
Keep going up and soon you’ll be able to enter a house with a blue roof.
Speak to the elderly man inside and he will ask you to show him a Pokémon of a particular level. The specific level he asks for is random and changes daily.
If you have a Pokémon of this level in your party, he will become ecstatic and award you with an item.
- The first time you show him a suitable Pokémon, he will give you a Black Belt
- The second time he will give you an Expert Belt
- The third time he will give you a Focus Sash
This can be repeated infinitely, and the cycle starts over after you receive all three items for the first time.
Expert Belt Details and Uses
The Expert Belt is a Hold Item that boosts the power of super-effective moves by 20%.
The ideal Pokémon to hold an Expert Belt is one with a moveset that features damaging attacks of a wide variety of types.
For instance, consider a Mamoswine with these moves:
- Ice Beam
- Knock Off
It would have super-effective options against almost every single Pokémon typing in the game.
Because of the large number of variables when it comes to type-matching, Expert Belts can be a challenge to optimize.
A simpler alternative for a held item would be to use something like the Life Orb, which raises the power of all moves by 30%, but at the cost of 10% of the user’s maximum HP with every attack.