I feel like if this ever happened, you’d want to practice that first step, because it might be easier to get gallons of urine then extract from a number of those sources.
So just to start on your first point, let’s assume the flare hit and you need to start making black powder from raw ingredients. Since there are no more available with supply chain breakdown, without consulting the internet (also down), where are those coming from within walking or bicycle distance with maybe a paper map if you still have one?