Okay, so potassium iodide is cheap and effective. Cool. What about iodine and kelp tablets from every grocery store? Aren’t those effectively the same thing?
KI is a thyroid blocker and is only good if you are exposed to early fallout from a nuclear detonation or accident involving fresh nuclear fuel in a reactor (the latter is very improbable). Any kind of release from decayed fuel will not have a significant radioiodine source term.
Use KI like any dietary supplement. The severe allergic reaction rate from literature I've reviewed ranged from 0.5 to 2 per million people. Pretty low, but if it were me I'd check with my family doctor first before considering stocking it.