If you want merch that comes straight out of Pokemon Sun and Moon, this Z Ring is a great choice. It can connect with the Sun and Moon games, so that pressing the gem in the actual ring will activate a Z-Move in-game.
This ring can also be used to make a costume based on the game's main characters, who adventure all over the Alola region in an effort to catch 'em all. Though you'll often find this product in brick-and-mortar stores like Toys-R-Us, they don't always come with the full set of Z Crystals. So ordering online is likely your best bet.