Where to catch Koffing

In Kitakami (Scarlet & Violet – The Teal Mask)

• Crystal Pool — Walk
• Infernal Pass — Walk
• Oni Mountain — Walk

In Hoenn (Ruby, Sapphire & Emerald)

• Fiery Path — Walk

In Johto & Kanto (Gold, Silver & Crystal)

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

In Kanto (Red, Blue & Yellow)

• Pokémon Mansion — Walk

Koffing — full Pokédex entry