Offhand, I don't recall any light xbow drops from critters. Usually the best place in my games was Anna in Arnika. And yes, the game seems to know what you're looking for and won't show it
I recall having a hard time myself getting that third xbow when I had a gadgeteer who could make one. Typically I would save before talking to her and reload if she didn't have a light xbow to sell. However, that was the original release with only the AP patch. I think it's different with later patches, and you just have to camp and keep trying. Good luck.
Nightowl2