Powerfull anti-venom is a reputation reward from Argent Dawn. U need to be friendley if I am correct. If u are friendley, u can get it in Eastern Plaguelands, at the Light`s Hope Chappel (there is a vendor that sells reputation rewards). I dont think u can find the recipe on AH, cause its a BoP recipe, so u have to become friendley and buy it at the vendor.