Here's a breakdown of where to find some common and notable Pokémon in FireRed, organized by location and level range:
Starting Area:
* Route 1, Viridian Forest (Levels 1-5): Pidgey, Caterpie, Weedle, Rattata, Spearow
* Route 22 (Levels 5-10): Mankey, Pidgey, Spearow, Rattata
* Viridian Forest (Levels 7-15): Pidgey, Caterpie, Weedle, Oddish, Bellsprout, Mankey, Spearow, Rattata, Pikachu (rare), Ekans (rare), Sandshrew (rare)
Early Game:
* Route 3 (Levels 10-15): Pidgey, Spearow, Rattata, Ekans, Sandshrew
* Route 4 (Levels 12-18): Mankey, Pidgey, Spearow, Rattata, Ekans, Sandshrew, Meowth (rare)
* Route 24 (Levels 10-15): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male)
* Route 25 (Levels 14-18): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male)
* Route 2 (Levels 12-18): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male), Growlithe (rare)
Mid-Game:
* Route 6 (Levels 16-24): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male), Growlithe (rare), Vulpix (rare), Ponyta (rare)
* Route 7 (Levels 20-25): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male), Growlithe (rare), Vulpix (rare), Ponyta (rare), Venonat (rare)
* Route 8 (Levels 22-28): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male), Growlithe (rare), Vulpix (rare), Ponyta (rare), Venonat (rare), Machop (rare)
* Route 9 (Levels 25-30): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male), Growlithe (rare), Vulpix (rare), Ponyta (rare), Venonat (rare), Machop (rare), Golbat (rare)
* Route 10 (Levels 28-34): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male), Growlithe (rare), Vulpix (rare), Ponyta (rare), Venonat (rare), Machop (rare), Golbat (rare), Graveler (rare)
* Route 11 (Levels 30-36): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male), Growlithe (rare), Vulpix (rare), Ponyta (rare), Venonat (rare), Machop (rare), Golbat (rare), Graveler (rare), Drowzee (rare)
* Route 12 (Levels 32-38): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male), Growlithe (rare), Vulpix (rare), Ponyta (rare), Venonat (rare), Machop (rare), Golbat (rare), Graveler (rare), Drowzee (rare)
* Route 16 (Levels 36-42): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male), Growlithe (rare), Vulpix (rare), Ponyta (rare), Venonat (rare), Machop (rare), Golbat (rare), Graveler (rare), Drowzee (rare), Geodude (rare), Hitmonlee (rare)
* Route 17 (Levels 34-40): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male), Growlithe (rare), Vulpix (rare), Ponyta (rare), Venonat (rare), Machop (rare), Golbat (rare), Graveler (rare), Drowzee (rare), Geodude (rare), Hitmonlee (rare), Hitmonchan (rare)
* Route 18 (Levels 40-46): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male), Growlithe (rare), Vulpix (rare), Ponyta (rare), Venonat (rare), Machop (rare), Golbat (rare), Graveler (rare), Drowzee (rare), Geodude (rare), Hitmonlee (rare), Hitmonchan (rare), Kangaskhan (rare)
* Route 23 (Levels 38-44): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male), Growlithe (rare), Vulpix (rare), Ponyta (rare), Venonat (rare), Machop (rare), Golbat (rare), Graveler (rare), Drowzee (rare), Geodude (rare), Hitmonlee (rare), Hitmonchan (rare), Kangaskhan (rare)
* Mt. Moon (Levels 15-30): Geodude, Clefairy, Zubat, Paras, Machop, Clefairy, Nidoran (female/male), Ponyta, Vulpix
Late Game:
* Route 15 (Levels 42-48): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male), Growlithe (rare), Vulpix (rare), Ponyta (rare), Venonat (rare), Machop (rare), Golbat (rare), Graveler (rare), Drowzee (rare), Geodude (rare), Hitmonlee (rare), Hitmonchan (rare), Kangaskhan (rare), Pinsir (rare), Scyther (rare), Electrode (rare)
* Route 19 (Levels 44-50): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male), Growlithe (rare), Vulpix (rare), Ponyta (rare), Venonat (rare), Machop (rare), Golbat (rare), Graveler (rare), Drowzee (rare), Geodude (rare), Hitmonlee (rare), Hitmonchan (rare), Kangaskhan (rare), Pinsir (rare), Scyther (rare), Electrode (rare), Onix (rare)
* Route 20 (Levels 46-52): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male), Growlithe (rare), Vulpix (rare), Ponyta (rare), Venonat (rare), Machop (rare), Golbat (rare), Graveler (rare), Drowzee (rare), Geodude (rare), Hitmonlee (rare), Hitmonchan (rare), Kangaskhan (rare), Pinsir (rare), Scyther (rare), Electrode (rare), Onix (rare), Rhyhorn (rare), Magmar (rare)
* Route 21 (Levels 48-54): Pidgey, Spearow, Rattata, Ekans, Sandshrew, Nidoran (female/male), Growlithe (rare), Vulpix (rare), Ponyta (rare), Venonat (rare), Machop (rare), Golbat (rare), Graveler (rare), Drowzee (rare), Geodude (rare), Hitmonlee (rare), Hitmonchan (rare), Kangaskhan (rare), Pinsir (rare), Scyther (rare), Electrode (rare), Onix (rare), Rhyhorn (rare), Magmar (rare), Alakazam (rare)
* Power Plant (Levels 50-56): Magnemite, Voltorb, Electabuzz, Magneton
* Seafoam Islands (Levels 50-56): Shellder, Staryu, Krabby, Dewgong, Cloyster, Seel
* Cinnabar Island (Levels 52-58): Grimer, Muk, Koffing, Weezing, Slowpoke, Slowbro, Voltorb, Electrode, Rhyhorn
* Victory Road (Levels 54-60): Onix, Geodude, Graveler, Rhyhorn, Golbat, Rhydon, Dugtrio, Arcanine, Snorlax
Legendary Pokémon:
* Articuno: Found in the Seafoam Islands (requires Surf)
* Zapdos: Found in the Power Plant (requires Surf)
* Moltres: Found in the Victory Road (requires Surf)
* Mewtwo: Found in the Cerulean Cave after beating the Elite Four (requires Surf and Flash)
Important Notes:
* Rare Pokemon: The rarity of some Pokemon like Pikachu, Growlithe, and Vulpix can vary, so you may need to try a few different encounters.
* Level Variation: Pokemon levels can fluctuate slightly depending on your progress in the game.
* Repel: Use Repel to avoid battling lower-level Pokémon while searching for specific ones.
* Breeding: Once you reach the later stages of the game, you can breed specific Pokémon for better stats.
* Trading: Trading with other players can help you complete your Pokédex.
This information should be a good starting point for finding Pokemon in FireRed. Remember to experiment and explore to find the Pokémon you're searching for!