jiansonz on 9/4/2014 at 07:48
Calendra's Cistern has a poison vial that technically works like holy water, but instead of hurting undead it will turn your water arrows into deadly poison arrows for 30 seconds.
Ultima Ratio has a single poison arrow that is used in one of the objectives.
Azaran on 9/4/2014 at 07:51
Mirror of Return has Zombie arrows. I think there's a couple of others that have them if I'm not mistaken.