Don't just focus on potassium iodide. Yes, it's useful, but only helps a specific condition, which is radioactive iodine settling in the thyroid (and probably other areas of the body with concentrations of iodine, such as the gonads and breasts).
Boron is capable of absorbing radioactivity in general. Buy a couple bottles of boron supplements and plan to use those to help pull radioactive isotopes from your body. Heck, you could buy a box of 40-Mule Team, which is cheap and 99% pure borax (no, it's not poisonous).