Hand warmers can be bought at many Walmart and/or sporting goods stores in states where the temps. get low and there is demand for them in hunting season. There are several brands [ Hothands is one ] and like was mentioned, they REALLY get hot, so you would need a good towel around them. They are relatively cheap, and you can get ones that last 12 - 20 hours. For packs that last a long time, you can Google " hot packs for shipping " and get ones online that last 60 hours, possibly even 72. You might find them at pet stores or reptile expos as well.