A Dragon Scale is an item introduced in Generation II that evolves Seadra into Kingdra when held while being traded. This item cannot be bought, only found, but can be sold for 1050.
In the Johto region games, this item can be found in Mt. Mortar and at the Pokéathlon Dome in Pokémon HeartGold and SoulSilver. It can also be held by wild Dratini, Dragonair, Horsea, and Seadra. In the Hoenn region games, it can be held by wild Horsea or Bagon. In Pokémon FireRed and LeafGreen this item can be found being held by wild Horsea or at the Water Tower. In the Sinnoh region games, it can be found held by wild Horsea, Seadra, Dratini, Dragonair, or even wild Bagon. In the Unova region games, it is found on Route 18.
Omega Ruby and Alpha Sapphire: Can be found at the Sky Pillar.