You are correct. Zyklon B is not readily available, thank God or whomever else might be responsible for that. However, the ingredients for poison gas and bombs as means of mass murder are. The Boston Marathon bombers are good examples of what destruction can be created with readily available materials. I suspect anyone with a good understanding of biological chemistry could mix up a batch of lethal gas. Even some commonly on hand household items, bleach and cleanser for instance, in combination can be lethal.