Crown of Fruity Pebbles can be located on the first floor of the Clothing Store. They can be purchased for 17 Bucks. After purchasing, you'll only be able to wear them in Adopt Me!, unless you buy them in the Roblox Catalog.
This item's descroption on the Roblox Catalog is:
"This is advertising. But also a reward for your skill – you know its thriller. Plus everyone knows to respect the Crown. Hail to the King, baby."