Sibbi BlackBriar, that guy in the jail: his chest is at BlackBriar Lodge, the same place you go to pull shenanigans with a horse (steal it, i think? can't remember).
There's a female BlackBriar walking around Riften, she *also* has a chest, that resupplies every once in a while with alchemic ingredients. This happens after you do her quest to get 20 nirnroot/deathbell/nightshade. That chest is in the back room of the alchemy shop.