There are oodles of internets sites just dying to sell you your favorite iodine supplement. A quick search will reveal more than you can shake a stick at.

The idea is to saturate your thyroid gland with iodine to keep it from absorbing radioactive isotopes of iodine that are in nuclear fallout. This is to help prevent thyroid cancer, and actually works for that purpose. It does nothing at all to protect you from the many other radioactive elements in fallout or a dirty bomb, and in my opinion, is basically being used as a government placebo to make people feel more secure.

I've got some surplus geiger counters and dosimeters, available on e-bay and elsewhere in great supply, since our brilliant government dismantled our civil defense system and sold off all the supplies as junk.