It can only be found off the carriers/messenger guys(forgot what they're called) that spawn randomly throughout the game. A easy way to farm them is to take a tunnel to the thieves guild, then climb on top of the tunnel and face the road heading into that
area. Within a minute or two you should see one of those guys coming down the road, then kill or tackle him. Then take a tunnel someplace else, then take it back to the thieves guild and repeat until you get it.
I went through my entire first playthrough (40+ hours) without ever finding aconite. I'm currently on my second playthrough (I raged quit hard on multiplayer and didn't touch the game for months) and just found it on a messenger. It's always a good idea
to wait awhile on the theif missions because of the useful items on the Hundred Eye gang.