Where to catch Shellder

In Paldea (Scarlet & Violet)

• East Paldean Sea — Walk
• North Paldean Sea — Walk
• South Paldean Sea — Walk
• West Paldean Sea — Walk

In Johto & Kanto (Gold, Silver & Crystal)

• Cinnabar Island — Good Rod, Super Rod, Walk
• New Bark Town — Good Rod, Super Rod
• Olivine City — Super Rod
• Pallet Town — Good Rod, Super Rod
• Route 26 — Good Rod, Super Rod
• Route 27 — Good Rod, Super Rod
• Sea Route 20 — Good Rod, Super Rod
• Sea Route 21 — Good Rod, Super Rod
• Sea Route 41 — Good Rod, Super Rod
• Vermilion City — Good Rod, Super Rod

In Kanto (Red, Blue & Yellow)

• Cinnabar Island — Super Rod, Walk
• Route 11 — Super Rod
• Route 17 — Super Rod
• Route 18 — Super Rod
• Route 6 — Super Rod
• Sea Route 19 — Super Rod
• Sea Route 20 — Super Rod
• Sea Route 21 — Super Rod
• Seafoam Islands — Super Rod
• Vermilion City — Super Rod

Shellder — full Pokédex entry