Walkthrough
After saving Mr. Fuji from Pokemon Tower, you really have two options moving forward. You could train by heading south of Lavender Town on Route 12 towards the next Pokemon Gym in Fuchsia City, or you could attempt to defeat Team Rocket at the Silph Co. Building in Saffron City. The wiser choice of the two would be to head on Route 12 towards Fuchsia City, allowing your Pokemon Team to get all the more stronger before having to defeat Team Rocket once again.
Wild Pokemon
Grasses:
Oddish(FireRed)/Bellsprout(LeafGreen) (35%): Levels 22-26
Pidgey (30%): Levels 23-27
Venonant (30%): Levels 24-26
Gloom(FireRed)/Weepinbell(LeafGreen) (5%): Levels 28-30
Surfing:
Tentacool (100%): Levels 5-40
Magikarp (100%): Level 5
Horsea(FireRed)/Krabby(LeafGreen) (60%): Levels 5-15
Krabby(FireRed)/Horsea(LeafGreen) (20%): Levels 5-15
Magikarp (20%): Levels 5-15
Horsea(FireRed)/Krabby(LeafGreen) (84%): Levels 15-35
Gyarados (15%): Levels 15-25
Psyduck(FireRed)/Slowpoke(LeafGreen) (1%): Levels 25-35
Poke Flute:
Snorlax: Level 30
Pokemon Trainers
Fisherman Ned: Level 22 Goldeen, Level 22 Poliwag, Level 22 Goldeen
Fisherman Chip: Level 24 Tentacool, Level 24 Goldeen
Fisherman Hank: Level 27 Goldeen
Fisherman Elliott: Level 21 Poliwag, Level 21 Shellder, Level 21 Goldeen, Level 21 Horsea
Young Couple Gia & Jes: Level 24 Nidoran Female, Level 24 Nidoran Male
Rocker Luca: Level 29 Voltorb, Level 29 Electrode
Camper Justin: Level 29 Nidoran Male, Level 29 Nidorino
Fisherman Andrew: Level 24 Magikarp, Level 24 Magikarp
Pokemon Trainers - VS Seeker Rematch
Fisherman Elliott: Level 28 Poliwhirl, Level 28 Cloyster, Level 28 Seaking, Level 28 Seadra
Young Couple Gia & Jes: Level 30 Nidorina, Level 30 Nidorino
Young Couple Gia & Jes: Level 55 Nidoqueen, Level 55 Nidoking
Rocker Luca: Level 33 Electrode, Level 33 Electrode
Items
Iron On Ground
TM27: On Ground
TM48: On Ground
Hyper Potion: ItemFinder
Leftovers: ItemFinder
Rare Candy: ItemFinder
Net Ball: From Fishing Guru Brother
Super Rod: From Fishing Guru Brother