Diamonds Are a World's Best Friend
Super strong woven armour
Impossible to buy outside of the black market. Extremely light, woven armour. Impervious to arrows, stabbing weapons and light slashing weapons. One step up from Mithril because of the weight to strength ratio. Since it is a fabric it can come in many different forms, vest, bracers, socks, blankets and others.
Myelin is found explicitly on kingdom courriers. Making myelin requires a direct order from the ruling authority and the ruler only gives the order when someone is exaulted to the status of courrier, so there is never any surplus. The only two ways to get a piece of myelin armour is to either become a courrier or kill one and take theirs. Courriers will defend their myelin to the death and the international punnishment for killing a courrier is death by appendage removal.