Sandslash locations — where to catch

In Kitakami (Scarlet & Violet – The Teal Mask)

• Paradise Barrens — Walk
• Reveler's Road — Static Encounter

In Sinnoh (Diamond, Pearl & Platinum)

• Route 228 — Walk

In Johto & Kanto (HeartGold & SoulSilver)

• Johto Safari Zone — Walk
• Mt. Moon — Walk
• Route 26 — Walk
• Route 27 — Walk

In Kanto (FireRed & LeafGreen)

• Route 23 — Walk

In Johto & Kanto (Gold, Silver & Crystal)

• Mt. Moon — Walk
• Route 26 — Walk
• Route 27 — Walk

In Kanto (Red, Blue & Yellow)

• Cerulean Cave — Walk
• Route 23 — Walk

Other games (7)

• Brilliant Diamond / Shining Pearl (Gen 8): Kanto
• Sword / Shield — The Isle of Armor (Gen 8): Challenge Beach, Courageous Cavern, Forest of Focus, Max Lair, Potbottom Desert
• Ultra Sun / Ultra Moon (Gen 7): Kanto
• Sun / Moon (Gen 7): Kanto
• X / Y (Gen 6): Kalos Route 18, Terminus Cave
• Black 2 / White 2 (Gen 5): Relic Castle, Unova Route 15
• Black / White (Gen 5): Relic Castle

Sandslash — full Pokédex entry