FWIW, that Oathkeeper comes from a bootleg set you can find very cheaply most places (here it is for $9 on Amazon, and there are some alternate versions with the crown/cross pendants).
Unfortunately I'm not sure offhand what the best option would be? The official necklaces are small, pricy, and don't include the Oathkeeper or paopu IIRC, and licensed Keyblade stuff isn't really great necklace material (like this). I think I've seen some cute fanmade enamel pins of Keyblades recently - check Storenvy, maybe? This person has some, it looks like.