Are you talking about this scale?
You need to get this scale by defeating/capturing the Red Gyarados in the Lake of Rage. It is Level 30.
After defeating/capturing it, talk to Lance in the Lake of Rage. He'll say some shtuff about how Gyarados was forced to evolve by some mysterious radio broadcast. He then proceeds to fly away on his hacked Dragonite, and he'll say he's waiting for you in the Souvenir Shop of Mahogany Town.
After going there, the entrance to the Rocket HQ will be revealed, and Lance will be waiting for you right inside.
Source: Experience and this.