So obviously there are plenty of poisons that occur naturally in a lot of different creatures, plants and such. In looking into the RAW there doesnt appear to be a way to harvest it but the closest things would be either survival(To know how to harvest the poison/not die doing so) or through heal as that covers surgeries and such things.
Are any of those actually a viable option to be able to gather poison to be used at a later time or do all poisons need to be bought/created?
In the past I've run games where characters with the right skills harvested parts and fluids from monsters (only those known to be alchemy/poison precursor materials) and sell them (i.e. loot in gp form). Its certainly good role playing in any case. It requires a bit of skill though. You have to know what is valuable, know how to get it, know who to sell it to and negotiate a decent price...it stimulates a bit of role play so it cant be all that bad. In fact, a whole new adventure could be based of of selling rather rare (read somebody is going to probably try to take them away from you) and valuable stuff. Kind of like drug running but in this case probably legal (or maybe not depending on where you are).