One important thing I forgot to mention is that crafting poison, you progress in gold per week of work rather than silver. That lets you craft it in a reasonable time. There is also a few different magical workshops you could use to boost up your productivity, and some feats that lower the cost and time.
The only important part to collect for posionmaking is the active ingredient, which should usually be obvious by the name, ie monstrous centipede poison clearly would be harvested from monstrous centipedes, or black lotus from that kind of flower, etc. If you have a source of the active ingredient, then the cost for raw materials is 1/6th the market price instead of 1/3. If it is hard to find/buy in your area, then the cost of materials is 3/4 market price or possibly not even avalible at all.
Now where/how to find black lotus or monstrous spiders is a matter of the campaign/dm, so that will change. Collecting venom from a corpse or whatever should just be part of the posionmaking skill I would think, if you fail a craft skill by 5 or more then you mess up your raw materials. Also remember that you have a 5% chance of accidently poisoning yourself without Poison Use.
2 of the main spots for rules on crafting poisons are page 25 of Song and Silence and page 44-45 of Vile Darkness. There is also some feats and items, etc that will be useful if you keep your eyes open.
One time DMing Harbinger had a character who was into making poisons, so I included lots of poison traps that if he disarmed right, he could save the poison for later use. He also had some clay jars that you could throw and affect everyone with a fine powdered poison in a 10-foot spred, I think the rules for that type of poison is just in the DMG. And I also had a couple homebrewed poison recipes for him to find while adventuring to make some new custom poisons.