The Light Crossbow can be purchased from the Nomadic Merchant (Castle Morne Rampart) in eastern Weeping Peninsula for 1,500 Runes. Check out the map below for the exact location.

“Large crossbow made from composite materials ... The Arbalest can be looted from a corpse sitting on a rampart in the southern part of Stormveil Castle. Check out the map below for the exact ...