With a little time spent searching, you will quickly find threads and resources to aide with using the deep web/dark net for procuring opioids. A "venom," generally speaking, would not be recommended due to pain and other complicating factors.
As for your second question: if hanging is a reliable method, and if a "lethal" cocktail is reliable, then surely both of them combined will be doubly effective. However, your rate of success will be largely determined by your successful application of either method in question.