the first chance to get this item is in the order of the holy seal, its in your room in a chest.
(you will find it on other places later on.)
you can only access this room when you join this guild.
if you got 500 ducats, you can ask mattheo in san pasquale to customize it.
if you haven't got the money, wait until you're in the innercity where you can buy a smithy for mattheo and customize it then.