There are two ways to get a Shell Bell in Pokemon Scarlet and Violet. Since this item is arguably the best option for most Tera Raid Battles, you might wish to know where to get it. The first and best method overall would be to purchase this Hold Item from Delibird Presents in Levinicia. Other cities with Delibird Presents won't have it, so keep that in mind.
Alternatively, you can get a Shell Bell by talking to the Pokemon League Representative in Area Three's South Province after defeating seven nearby Trainers. This method can only be done once, whereas the Delibird Presents option is available forever.
How to get a Shell Bell in Pokemon Scarlet and Violet for Tera Raid Battles
Let's start with the easier method that requires minimal effort on the reader's part. Simply go to Levincia's northern Pokemon Center and go slightly southeast to the Delibird Presents pictured above. Going here and purchasing the item should only take you about a minute in Pokemon Scarlet and Violet.
To start, head inside the store as pictured below.
Go to Battle Items, and you should see Shell Bell on the first screen. One copy costs ₽20,000. Trainers do not need to collect any Gym Badges in order to be able to purchase this item. You may also use League Points instead of money if you lack the latter currency.
Either way, purchase as many copies of this Hold Item as you think you'll need.
The alternate method of getting a Shell Bell is to go to the above location and fight seven Trainers before talking to the Pokemon League Representative. If you don't know where this spot is, it's east of Mesagoza, near the closest Pokemon Center.
It doesn't matter which Trainers the player defeats in Pokemon Scarlet and Violet in Area Three's South Province as long as they defeat seven of them.
Why Shell Bell is the best item for Tera Raid Battles
One of the most commonly recommended items for Tera Raid Battles in Pokemon Scarlet and Violet is the Shell Bell. The reason is simple: you can heal a ton of HP whenever you're dealing damage against bosses with a ton of HP.
Here is this item's description in Pokemon Scarlet and Violet:
"An item to be held by a Pokémon. The holder restores a little HP to itself every time it inflicts damage with an attack."
This item is much better in Tera Raid Battles than PvP since the opponents in the previous bouts have significantly more HP. Sustain is the name of the game for this type of content, so being able to withstand heavy hits and deliver them back to heal most of (if not all) the damage with the right strategies is incredible.
There are only a few cases where one would forego a Shell Bell in favor of a different item in high-rarity Tera Raid Battles.