How to ObtainEdit
The Mettenheim Arbalest can be used by your Custom Knighthood Order, however, it needs to be unlocked. To do so, you must belong to any Knighthood Order (even your own counts), be rank 5 or higher (Knight Commander - see ranks here) and talk with the knight once your crossbow proficiency reaches 300. The knight will grant you a prototype of the weapon and will also unlock it for your Custom Knighthood Order to be equipped.
The only way to obtain another Mettenheim Arbalest is as a very rare drop from the only troop that uses it, the Mettenheim Heavy Crossbowmen, this however will not allow your CKO to use them.