I rember at one time having one of those hand warmer packet things, but this one activated by "clicking" what felt like a small metal wafer inside the pack. It would be HOT for 5-6 hours and still good and warm after 8. It was reusable. Just put in in boiling water to "reset" the chemicals inside. Really heavy plastic pouch. If you could find a few of those tou should be set. My guess would be like an Army/Navy store or a good sportsmans catalog. Mike