IOSAT is regulated (unlike supplements) by the FDA. Expiration dates, it's facilities, and dosage levels are regularly tested for efficacy.
IOSAT Potassium Iodine (iodide) comes in 130 mg tablets. A 130 mg tablet is enough to keep adults and kids over 12 (150 lbs or more) protected from radioactive iodine for 24 hours. The IOSAT packs (14 tablets per pack) have detailed dosage information for adults, children, and infants.
You can also buy cheaper alternatives. They aren't regulated (sold as vitamin supplements), but they are still an alternative to the more expensive Government issued IOSAT tablets.