Pokémon FireRed and LeafGreen/Three Island

Three Island (or Kin Island) is the third island in the Sevii Islands. The port is the home of Dunsparce.

Dunsprace
To find Dunsprace, you have to first defeat the Elite Four. Then you go through the cave at the start of the island. There are no Pokémon in the cave, so you don't have to worry about finding any unwanted guests. When you get to the other side, there is some grass at the other side. There is a 100% chance of getting the Dunsparce. Catch him, then go get the nugget hidden in the cave.