The only place to find guns (besides pillaging ships) is in the gunrunner's hideout. Gunrunning is a highly unpredictable science - in fact, no one knows exactly how to make them work: ending up with pieces of bricolated machinery that often explode (or jam) right in your face.
Unlike other "harbour merchants", these men are sharp businessmen. No fooling around, no haggling, no bartering. One price, one means of payment: cash (guns are rare, extremely expensive, and generally hard to get). They are upper magistrates who have come up with a smart way of earning an extra buck. Young, good looking, fancy dressers.
Money talks, bullshit walks.