* Route 42: After you beat the 4th Gym leader, Morty, and have access to Route 42, you can find Tentacruel in the water.
* Route 43: Another location to find Tentacruel.
* Route 44: You can also find Tentacruel in the waters along this route.
Note: Tentacruel is a water-type Pokemon, so you'll need to encounter them in water areas.
Remember, it's always a good idea to check the Pokemon locations guide for more specific information.