The magelo link @Redmonkey provided shows clearly that humanoids drop it. Just need to farm the hell out of humanoids for the drops.There is no good spot for this. /boo
Good news is that it is in global loot tables so any mob can drop it, only seen it from humanoid myself so there may be a restriction.
Bad news it is rare.
Best farming bet is find a 50-60 ish zone with fast humanoid respawns and just slaughter it for hours on end.
Or offer tons of plat in barter /laughter looking at my plat total.
And people think I am nuts for storing anything with a tradeskill tag.