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.
it needs to hold a dragon's scale and trade it
get a dragon scale and trade it then trade it back it will evolve
you have to catch a dragon Pokemon holding a dragon scale
you need to give seadra a dragon scale (which you can obtain by catching or using thief on some dragon Pokemon (bagon for instance) and theres a chance that they will be holding a dragon scale) and then you trade your seadra holding the dragon scale and voila you have a kingdra
make it hold the item "dragon scale" and then trade it
If held by seadra during a trade it will evolve to kingdra.
The Dragonscale is an item in Pokemon Emerald version. There is a chance that certain Pokemon such a Horsea will be holding one when you catch it.
It is in the middle cave of Mt.Mortar, it takes surf and waterfall to eventually get to it
go to lake rage and catch red gyrodos
catch a dragon Pokemon such as horsea and Bagon until you get one holding a dragon scale.
you get kingdra by trading a seadra that is hold "dragon scale" you can get dragon scale by looking in the bottom level of mount mortar you need the HMs Strength, Surf, and Waterfall