If you are near a feed or pet food store, shelled field corn will serve the same purpose and is lots less expensive, or at least it used to be.
Make the bags and fill with the shelled hard corn and then microwave them for 3-4 minutes and use for warming bags for your feet, back or wherever. My cats and little dog love them also when it is cold out :? !
Have used them for several years. When I heat them too long, it just smells like popcorn! :D