I'm pretty sure you get it by catching random dragon Pokemon and they will be holding it.
you have to catch a dragon Pokemon holding a dragon scale
Start fishing for Horseas in Route 226 and you should find one holding it.
if u have Pokemon platinum then just trade a Pokemon from your platinum version holding dragon palse.
let it hold a dragon scale and trade it then it will evolve(dragon scales are only found on seadras sometimes when you catch them that's the only way to find them)
a wild bagon may hold it
You can find one at six island in the south.
On six island go south and use surf you should find an item that's the dragon scale.
You will find the dragon scale deep in Mt. Mortar, which is between route 42 and Ecruteak City.
You can get it by going to the pokeatolon point shop.
Mt. Mortar or you can buy it in the Pokeatholon Dome
You can find the Heart Scale under ground by using the Explorer Kit.
Go to the lake of rage and catch or deafeat red gyarados and you will automatacly get the dragon scale^ That's a Red Scale. Trade it to Mr. Pokemon for the exp. share. The Dragon Scale can be found in Blackthorn City after defeating Claire.Capture a dragon type pokémon or receive the drantini in that cave near the dragon gym.You find it in Mt. Mortar in the Johto region. You need to have surf and waterfall available and you find it in the area where a school boy has a level 30-40 Seadra.