If one person very accustomed to smoking marijuana survive a zombie apocalypse, probably at first is devoted to survive, find food, seek shelter and fortify this, etc. In addition, 99.9% of cops have become zombies, so the drug law no longer exists. And if it is not allowed, it is equally cultivated. Why not grow marijuana ourselves? In the real world you can, if permitted in your country.
Marijuana appeared out of nowhere, and if you wanted to smoke more, you depended to find more joints or marijuana of fallen zombies or looted houses. However, in my point of view, this a bit unrealistic.
Some friends asked me to create a mod for smoking joints, and I knew that already on some outdated mods like Apocalypse +, a marvelous mod, you can do that.