I suggest Iodophor. B-T-F Iodophor is what is most commonly stocked at homebrew supply stores, and it's what I use. 3 ml (slightly more than a half teaspoon) per gallon of water makes the recommended 12.5 ppm solution. As rjsnau mentioned, a pint bottle of it will last you a very long time. A quart should last you several years.