Turns out you have to get the venom sac from those damn dune spiders in North Ro. >.> Fuuuuuuuuu-
If you have other purposes for them, you'll have better luck farming this in the Commonlands; I found mine at 149.69, 385.01, 121.36 on a giant spider; I also saw a spider at the small tower guarded by two skeletons and another at the plagued huts.
Or you might just want to buy them from the Cleric/Paladin spell merchant in Crescent Reach... he/she was selling about 10 when I got there to dump my vender trash.
Edited, Apr 2nd 2012 9:29pm by Irrintra
Edited, Apr 2nd 2012 9:33pm by Irrintra