I don't think anyone has mentioned things that could potentially taken from creatures that live in many fantasy worlds and as such they could be considered "naturally occuring"... imagine hiring a dragon that has toxic gas for a breath weapon and "bottling it" and then boiling it down for the extract... imagine arrows or jars of the chemical used in catapults. Extracting the venom from a chimera's throwable quills could work too, a paralytic agent that could be very effective as a biological weapon in battle or during a seige.
You could very well have someone cut open the "flame glands" of a dragon and extract the liquids within. Could be a good subplot, some guy is hired to kill a dragon not to save the villagers but to fuel some sorcerer's flamethrower...