Walkthrough
Once you have successfully obtained the first six Kanto Gym Badges, the next place to logically head is south of Fuchsia City on Route 19. However, since you will need HM Surf to do so, you could actually take a break from your Gym Challenge in order to search for your first Legendary Pokemon. If you wish to do so, head back to Route 10 (east of Cerulean City and Route 9). Now that you have the ability to surf on waters, you can do so Route 10. By riding the waves of Route 10, you will quickly find yourself at the Power Plant. This building is pretty deserted, only having a few Wild Pokemon within it. However, at the very end of the Power Plant, you will find a Legendary Pokemon!
Wild Pokemon
Walking:
Magnemite (30%): Levels 22-25
Voltorb (30%): Levels 22-25
Pikachu (25%): Levels 22-26
Magneton [15%(LeafGreen)/10%(FireRed)]: Levels 31-34
Electabuzz(FireRed) (5%): Levels 32-35
Items
Max Potion: On Ground
TM25: On Ground
TM17: On Ground
Thunderstone: On Ground
Elixir: On Ground
Max Elixir: ItemFinder
Thunderstone: ItemFinder