the 7 island Pokemon tower owner may give you it.
Bagon carry it on them. (5% chance though)
give sedra a dragon scale and then trade it . it will be kindra when you trade it
Go to six island and go south use surf and go down some more until you reach a little platform with an item this is the dragon scale.
you have to catch a dragon Pokemon holding a dragon scale
catch a dragon Pokemon such as horsea and Bagon until you get one holding a dragon scale.
To get a dragon scale is to catch a horsea or a bagon.
The dragon scale is located in Mt. Mortar
You can evolve Seadra by trading it with the Dragon Scale
The Dragon Scale is in the trainer tower at the top of island 7. you must beat the trainer tower on doubles or beat it twice. Also there is one located near in ruin valley on six island need surf to get it.
And on leafgreen, and any other pokemon game, horsea evolves at level 40. Seadra evolves if you make it hold a dragon scale and than trade.
It only evolves If you trade it with a dragon scale
after you get the national dex you can give it to a seadra and trade it to get a kingdra.