(hectorshardware.com) has them. I couldnt find much info on the product on the internet. The chemical mats that are used last 4 hrs and the butane cartridge lasts 12hrs. Not sure if you can continue to use the same mat if it isnt used the entire 4hrs at once,maybe it can be wrapped in foil or something. Doesnt work unless you are in the same spot for at least 15min.