At the end of the Tree Gnome Village quest, as a reward; Add or edit an NPC to have an option to view their shop which sells an un-equipable Ballista that costs 250k per part with 4 total, very similar to Dwarf Cannon except it uses javelins/spears for ammunition.
TL;DR:It's a dwarf multicannon re-skinned with a different already existing game model (Pictured here: https://ibb.co/K2HXt3s) that uses a different item ID than cannonballs as ammunition.