I use regular POPCORN - I've made a bunch of pocketwarmers using 6" squares of old denim jeans. Right sides together sew around 3 edges (2 long - 1 short), turn right side out, fill with POPCORN, turn down top edge and sew closed on machine.
Heat for 1 minute (or a little more - you have to test your microwave).
Easy to find - pretty inexpensive - smells good when heated - great gifts for those who work in the cold.