The weapons-type trade good
Crossbows is available from this port's market during World Clock 15th Century, Periods 3-4, or with an Adept of Age title from those periods equipped.
Which means there is one European port in the game with two Weapon type goods for sale, if you meet the requirements, making Plymouth great for building the skill
Weapons Trading. I found this to be the easiest (15-p3) Chrono title quest:
Develop sea lane to Syracuse
Source:
gvo.gamedb.info/wiki/?WorldClock/item (World Clock specific goods/items/recipes)
gvo.gamedb.info/wiki/?Quest/ChronoQuest (Chrono Quest list)