Sadly, there are no legitimate in-game ways to get the Soothe Bell again except in the Sun and Moon series, but there are always other methods. One of which is trading; there are a lot of people willing to trade a Soothe Bell or just give one out. I found many when I started searching in the discord servers or on Reddit. There's always the Pokémon item duplication glitch you could use, which is available for most Pokémon games. However, only do that if you're well versed with Pokémon since glitches might break the game. With that said, the item duplication is primarily safe, and maybe you should try it out.