It sounds like you're on the right track for your experiment. Potassium iodide might be a little bit tricky to get, and you may have to order it online. Usually big science labs get it from chemical suppliers, but in that case you would get a huge bottle and it would be very expensive. One reasonable chemical supplier you could try is the Carolina Biological Supply Company (call 1-800-334-5551, or go to their website and search for "potassium iodide" or follow this link :

Another use of potassium iodide is to protect your thyroid if you are exposed to nuclear radiation. According to the governments website, most pharmacies should have a supply of potassium iodide. If you live near a CVS or Walgreens or other pharmacy, you could ask the pharmacist to help you. In this case, the potassium iodide might come in tablet form and you may need to grind it up.

A third option is to consult your teacher about the possibility of asking a high school chemistry teacher to help you with your project. Many high school chemistry labs have common chemicals such as potassium iodide, so they might be able to supply you.

In any case, be sure to read the bottle or label of anything you're working with to make sure you use it safely. Good luck!