Things You'll Need
Instructions
Keep a lookout for Borgia couriers. They don't appear on your mini-map, but an arrow will appear above their heads when they're close, and they're always running, as opposed to every other character in the game who walks slowly.
Tackle the Borgia couriers. Hold the "L1" button and "X" to sprint after them and tackle them. They'll surrender their loot out of fear, and it's usually something valuable. Couriers are somewhat hard to find, but they're the most likely source of aconite. They don't always have it, but they usually have at least a tomato or nutmeg.
Travel to the Antico District. Be prepared to fight, because you will be attacked randomly by bandits wearing handkerchiefs as masks.
Kill any bandits you find and loot their bodies. Killing bandits is unavoidable, but looting their bodies almost always gives you some uncommon and rare materials for the shop quests, including aconite.
Find Chests. Chests can be somewhat difficult to spot, but they always contain valuable materials for shop quests. Chests are usually found on balconies, behind pillars and in underground tunnels.