I have one, for pain therapy, I bought several years ago at a natural food store. It is 5" x 12" (appx) the cover is either hemp or linen I can't recall which, it is soft. It has rice filling, and the instructions were to microwave 2 min. It can also be put in the freezer for cold therapy. I like the additional ideas here-both how to make them, and other ways these hand bags can be made.
I have seen smaller ones for eyes with flax seeds, and silk fabric.