Koffing locations — where to catch

In Kitakami (Scarlet & Violet – The Teal Mask)

• Infernal Pass — Walk
• Oni Mountain — Walk
• Paradise Barrens — Walk
• Wistful Fields — Walk

In Sinnoh (Diamond, Pearl & Platinum)

• Stark Mountain — Walk

In Johto & Kanto (HeartGold & SoulSilver)

• Burned Tower — Walk
• Johto Safari Zone — Walk
• Team Rocket HQ — Static Encounter, Walk

In Hoenn (Ruby, Sapphire & Emerald)

• Fiery Path — Walk

In Kanto (FireRed & LeafGreen)

• Celadon City — Surf
• Pokémon Mansion — Walk

In Johto & Kanto (Gold, Silver & Crystal)

• Burned Tower — Walk
• Team Rocket Base — Static Encounter

In Kanto (Red, Blue & Yellow)

• Pokémon Mansion — Walk

Other games (4)

• Sword / Shield (Gen 8): Dusty Bowl, East Lake Axewell, Giant's Cap, Giant's Mirror, Hammerlocke Hills, Motostoke Outskirts, Motostoke Riverbank, North Lake Miloch
• Let's Go, Pikachu! / Let's Go, Eevee! (Gen 7): Kanto Power Plant, Pokemon Mansion
• Omega Ruby / Alpha Sapphire (Gen 6): Fiery Path
• Black 2 / White 2 (Gen 5): Virbank Complex

Koffing — full Pokédex entry