Where do you get a soothe bell in pokemon diamond?

Answer 1

You get one when you help Cheryl get through the Eterna Forest. There is another later in the game, not sure where though. Sorry. If someone knows locations of Soothe Bells, beyond what I listed, please "improve" this answer.

Answer 2

The soothe bell is an item which can be found in Pokémon Diamond and Pokémon Pearl. You can find it in the Pokémon Mansion.