In route 221 (near pal park) there's a house a bit north-west of the honey tree in the route. Inside an old man will give you one of three belts everyday if you show him the level pokemon he asks for and don't worry if you don't have the level of the pokemon he asks for it changes everyday. The belts are:

Black Belt: Powers up fighting type moves
Expert Belt: Powers up super effective moves
Focus Sash: The holder doesn't faint when hit by a one-hit KO move. (not just fissure, sheer cold, gullotine and horn drill but also lets say a one hit KO critical hit)