Well, I'm not sure if this is what you're looking for, but Earth Therapeutics has these round gel disks that can be reused for both cold and hot therapy. For cold, just stick them in the freezer for a few minutes and they will stay cold for a good while (the more time in the freezer, the colder they will stay). To make hot, stick them in hot water for a few moments.
They're no where near "instant" hot or cold, but they are pretty small (3.5" diameter). And although they are mainly for home spa treatments, they are reasonably effective for other medical uses.