first of all, you need to catch a Bagon in meteorite falls. Then travel to Pacifidlog town and talk to the lady in the top right house. She will trade you a Horsea for your Bagon.
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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.
Its not at Battle Frontier, its at Pacifidlog Town
use surf on route 132 and fish using a super rod(AWAY from the currents!!!!!)
you get horsea on route 226
You can get a Horsea in Pokemon Silver by finding it at the Whirl Islands.