They're all over Kyushu

You could buy a gunsmith workshop. It makes even better variants of normal guns. But I don't know if they are only in the south as well.

Btw, has anyone found an Ōzutsu to buy? There is ammo for it in the shop but other than that I only see them during sieges. (Fun fact: Using them improves engineering)

Any city with a gunsmith workshop, iirc these are randomized, so you'll just have to go find one. Urban notables with something like 'the Gunsmith' in their name, if you look them up on the encyclopedia ingame you should be able to see the city they are in and zoom to it or track it from in that screen.

You'll find guns largely where they were introduced to Japan - Check the island on the far west, home of the Saikai. You'll find a fair number of guns (And their Gun Samurai are insanely good - skill of 270 unmodified at the final promotion).