In Diamond, Pearl and Platinum, the player can obtain this item by visiting an old woman in Hearthome City.
In Soul Silver and Heart Gold it can be found near the entrance of Union Cave on Route 32 behind a rock that can be smashed
In Black / White, it can be obtained by talking to a man in Driftveil city. Although you can obtain Shoal salt and Shoal Shells in Gen V from the treasure hunter by Undella Town, it doesn't get you a Shell Bell in any way.
In Black 2 / White 2, you must go to Driftveil City, visit the building right under the Pokémon Center, talk to the old man and have seen at least 70 Pokémon, or behind the eight Gym.
In X and Y, you must go to Dendemille Town, visit the building to the right of the windmill, and talk to the man inside after having seen at least 70 Pokémon in the Mountain Kalos Pokédex.