Actually, it makes sense in the long run that you get that little of a poison from X critter unless you have a herd of X critter to harvest it and that is all you are doing is farming the poisons out. It is assumed that you are harvesting the poisons from a dead critter so you lose some due to decay or leakage while ripping apart the critter in question.

The only way you might be able to get around it is by magically harvesting the liquid, but I don't know of any off the top of my head right now, nor do I know where to start. I think Bastion Press has a pretty good selection in the Posioncraft handbook, but I have to find it first... yeesh, way to come to the game unprepared... If I find anything that might suit what you need, I will post later

Knowledge (Appropriate skill for the creature in question) to determine the nature of the poison. Survival to get into the venom sac. Craft: Poisonmaking to extract the poison. I set the DCs pretty high, because this shit is hard to do, and if they messed it up too badly, they lost the dose ad best, or get poisoned at worst.

I would make it sort of a side-quest to find these certain areas that have poisonous plants, and make it a habit of collecting any you see. Poisonous nightshade, for example. Or just invent some plants! Since you're a Druid, you should have a high Survival score.

Or, buy seeds to a poisonous plant, hire a commoner to tend them for you, and cast "Plant Growth" or whatever it is--the one that makes plants grow strong for a year--on your darling poisonous plants. Voila!

Well here's what we have done as a house rule for the sessions with my group. To get the poison required from said monster it requires the following:

1x Dead Monster (to extract from)
1 d20 skill roll (typically Knowledge Nature, but depending on the creature, sometimes planes or other) roll DC 10+ monster's HD
1 dexterity check (dc 20) to avoid being poisoned. (n.b. if the dex check fails, then the poison sac/vessel is also ruined and no dosage is extracted)

Now ontop of that the DM rolls a d% to determine if the monster in question has sufficient venom/poison left. % roll is determined by size of monster (typically larger monsters have greater % of yielding poison, however not neccesarily true of some monsters) The % starts at 30 and is increased 5% per 4HD of the monster in question. Now here comes the important bit. For every poison attack that the monster in question used during the fight to kill it, reduce the % by 5.

So Eg: Your party is fighting a Giant Spider to gather venom. During the battle the spider makes 3 attacks that use its venom, reducing the % by 15. The spider has 20HD, which increases the % by 25. The final rolls to determine success here would be:

1d20 Knowledge Nature roll - DC 30
1d20 Dexterity Roll - DC 20 (to avoid being poisoned)
1d% roll by DM (30 - 15 + 25) = 40% chance venom/poison left

This particular technique has been very effective for us, as it reduces the need to buy expensive poisons, but also shows why the poisons are expensive in the first place!! because they are hard to get! It also makes it harder for your PC's to constantly use poisons, as it is simply too difficult to constantly hunt monsters for poison as well as adventure.