Where can you purchase Pyrethrum?

At a garden center, though it won't be labeled as such. Pyrethrum is a very safe (for mammals), plant-derived, natural insecticide that comes from certain chrysanthemum-type plants. It is an ingredient in many "bug dusts" and sprays. But you have to read the labels. By the way, if the label says " . . .rethins" or "cole resins", that is pyrethrum, too.