Instructions
Get Technical Machine (TM) 46 from the Team Aqua Grunt in Slateport Museum. Use TM46 on a Pokemon to make it learn Thief. Unless the Pokemon using the move is holding an item, this move will steal the opponent's held item. With this move, you won't have to catch a Pokemon to obtain a held item.
Fly to Mossdeep City and enter the northernmost house. Speak to the Fisherman inside the home and he will give you the Super Rod. You will encounter Horsea more often if you fish with the Super Rod instead of the Good Rod or Old Rod.
Go to Route 132, Route 133 or Route 134. These routes are located west of Pacifidlog Town. Check your Town Map if you have trouble finding one of these three water routes. Place the Pokemon that knows Thief as the first Pokemon in your party. Fish with the Super Rod.
Run away from every Pokemon except Horsea. Use Thief on any Horsea you encounter. Since only about five percent of wild Horsea hold the Dragonscale, you will probably have to use Thief on about 20 Horsea to get the Dragonscale.