Walkthrough
After defeating the Fuchsia City Gym, you will then need to head to Saffron City, the location of the next Pokemon Gym. In order to do this, you can head back up Routes 12-15 to Lavender Town, or you can travels Route 16-18 to Celadon City. Route 18 is just west of Fuchsia City, and it's a relatively small path that will almost immediately connect to Route 17. Only a few Wild Pokemon and Trainers can be found here.
Wild Pokemon
Grasses:
Pidgey (20%): Levels 33-38
Pidgeotto (20%): Levels 33-38
Doduo (20%): Levels 33-38
Rattata (15%): Levels 33-38
Raticate (15%): Levels 33-38
Dodrio (10%): Levels 33-38
Grass - Special Spawns:
Chansey: Levels 33-38
Waters:
Tentacool (30%): Levels 33-38
Staryu (30%): Levels 33-38
Magikarp (30%): Levels 33-38
Tentacruel (9%): Levels 33-38
Starmie (1%): Levels 33-38
Sky:
Pidgey (60%): Levels 3-56
Pidgeotto (35%): Levels 3-56
Pidegot (5%): Levels 3-56
Sky - Special Spawns:
Dragonite: Levels 3-56
Charizard: Levels 3-56
Articuno: Levels 3-56
Zapdos: Levels 3-56
Moltres: Levels 3-56
Trainers
Bird Keeper Wilton: Level 39 Fearow, Level 39 Dodrio
Bird Keeper Ramiro: Level 39 Pidgeot
Bird Keeper Jacob: Level 39 Farfetch'd, Level 39 Farfetch'd, Level 39 Farfetch'd
Obtainable Items
Poke Balls: Defeat Bird Keeper Wilton
Poke Balls: Defeat Bird Keeper Ramiro
Poke Balls: Defeat Bird Keeper Jacob
Elixir: Near Bird Keeper Jacob
Silver Razz Berries: Hidden In Yellow Flower Patch
Post Game
Walkthrough
After defeating the Elite Four and starting the Master Trainers Challenge, you will find a couple of Master Trainers on Route 18. In total, there will be three Master Trainers to defeat.
Trainers
Bug Catcher Jake (Master Trainer): Level 65 Caterpie
Bird Keeper Theo (Master Trainer): Level 80 Fearow
Bug Catcher Ryouta (Master Trainer): Level 75 Pinsir