Muk locations — where to catch

In Blueberry Academy (Scarlet & Violet – The Indigo Disk)

• Coastal Biome — Walk, Static Encounter

In Johto & Kanto (HeartGold & SoulSilver)

• Celadon City — Surf
• Johto Safari Zone — Surf
• Route 16 — Walk
• Route 17 — Walk
• Route 18 — Walk

In Kanto (FireRed & LeafGreen)

• Pokémon Mansion — Walk

In Johto & Kanto (Gold, Silver & Crystal)

• Celadon City — Surf
• Route 16 — Walk
• Route 17 — Walk
• Route 18 — Walk

In Kanto (Red, Blue & Yellow)

• Cinnabar Island — NPC Trade
• Kanto Power Plant — Walk
• Pokémon Mansion — Walk

Other games (5)

• Let's Go, Pikachu! / Let's Go, Eevee! (Gen 7): Kanto Power Plant, Pokemon Mansion
• Ultra Sun / Ultra Moon (Gen 7): Kanto
• Sun / Moon (Gen 7): Kanto
• X / Y (Gen 6): Friend Safari
• Black 2 / White 2 (Gen 5): Castelia Sewers, Hidden Grotto, Unova Route 9

Muk — full Pokédex entry