The last cauldron that remains is located on a farm now called Gahrron's Withering.  It is guarded by some of the Scourge's tougher minions - specifically, wraiths and ghosts.  They are not to be trifled with, $N.$B$BGahrron's Withering is far to the east of the Bulwark, close to where the Eastern Plaguelands begin; again, eliminate the cauldron lord present there for its key.  Here is your empty sample bottle; bring it to me filled.  May the shadows cloak you in safety for this dangerous mission, $c.