The lotus in Fungus Grove who drop this stuff summon, cast, and hit HARD.
The easiest way to get the lotus off them is to go to the little red Tamrocks just to the right from the tunnel exit from town.
Assuming you haven't murdered a ton of them (most people don't so they aren't cut out of the caller quests), you will be at least apprehensive to them so they will sell to you.
LOTS of people dump the lotus sap on the vendors in their village, and for all I know they may just sell the stuff as a matter of course.
"God created man from a handful of dust, and he created woman from that man's rib. And these two together were so stupid that they weren't on the planet five minutes when they managed to get a curse put on all future generations. Nice work." Pat Condell