You probably won't find anything more than "hand warmers" locally. For the last few years, all of the heat packs have been coming in from China. I'm not sure if there are any American manufacturers left. But you can get good quality heat packs from superiorshippingsupplies.com
I would stick with the 60 or 72 hour heat packs for shipping fish, just in case of delays in transit.
Mike Hellweg, President Emeritus
Missouri Aquarium Society, Inc.