Question

Where can I find a Soothe Bell in Platinum?

Answer 1

You can get 2 Soothe Bells in Platinum:

1. From Cheryl in Eterna Forest in the main story, you get this just by playing through the game as part of the storyline progression.

2. From a maid NPC in the Pokémon Mansion on Route 212.

This guide has screenshots showing you how to get both: https://guidestrats.com/pokemon-platinum-soothe-bell/

And here's a screenshot of the main NPC to speak with, courtesy of the guide above:

Answer 2

the Soothe Bell can be found in the Pokemon Mansion on Route 212. Talk to one of the maids and they give it to you.

Source: http://bulbapedia.bulbagarden.net/wiki/Pokémon_Mansion_(Sinnoh)

Answer 3

You can find on on the ground in Etern Forest and another one is given to you at Mr. Backlot's Pokémon Mansion.

Source