Use Thief or Covet on the Machamp the Black Belt Nob on Route 115. At the fifth battle and the battles after that, he will have a Machamp.
To get him to fight you again, you may try to go into the cave and then back out again a few times. It may help, not guaranteed to work.
You can get several of this item, all you have to do is get him to fight you again after the fifth battle and keep stealing the item.
Make sure that the Pokemon who uses Thief or Covet on another Pokemon does NOT hold an item before trying to steal one.
Chat with our AI personalities
by going to the village and talking to the Pokemon
The Black Belt boosts the attack of Fighting moves.
It can repel wild Pokemon upon use.
You can get a black belt from a man in a residence on route 221
In Hell next to the man in black