Tangela locations — where to catch

In Hisui (Legends: Arceus)

• Cottonsedge Prairie — Walk
• Crossing Slope — Walk
• Gapejaw Bog — Walk

In Sinnoh (Diamond, Pearl & Platinum)

• Great Marsh — Walk

In Johto & Kanto (HeartGold & SoulSilver)

• Fuchsia City — Headbutt
• Mt. Silver — Walk
• Route 12 — Headbutt
• Route 13 — Headbutt
• Route 14 — Headbutt
• Route 15 — Headbutt
• Route 18 — Headbutt
• Route 28 — Walk
• Route 44 — Walk
• Sea Route 21 — Headbutt, Walk

In Kanto (FireRed & LeafGreen)

• Cinnabar Island — Trade
• Sea Route 21 — Walk

In Sevii Islands (FireRed & LeafGreen)

• Treasure Beach — Walk

In Johto & Kanto (Gold, Silver & Crystal)

• Mt. Silver — Walk
• Route 28 — Walk
• Route 44 — Walk
• Sea Route 21 — Walk

In Kanto (Red, Blue & Yellow)

• Cinnabar Island — NPC Trade
• Safari Zone — Walk
• Sea Route 21 — Walk

Other games (7)

• Sword / Shield — The Isle of Armor (Gen 8): Forest of Focus, Max Lair
• Let's Go, Pikachu! / Let's Go, Eevee! (Gen 7): Kanto Sea Route 21
• Omega Ruby / Alpha Sapphire (Gen 6): Dewford Town, Route 114, Route 124, Route 125, Lilycove City, Mirage Spot Forest, Mirage Spot Island, Mirage Spot Mountain, Mossdeep City
• X / Y (Gen 6): Friend Safari
• Black 2 / White 2 (Gen 5): Giant Chasm, Hidden Grotto, Unova Route 13
• Black / White (Gen 5): Giant Chasm, Unova Route 13
• XD (Gen 3): Cipher Key Lair

Tangela — full Pokédex entry