Via Wikipedia: Air-activated
hand warmers contain cellulose, iron, water, activated carbon, vermiculite (water reservoir) and salt and produce heat from the exothermic oxidation of iron when exposed to air. They typically emit heat for 1 to 10 hours, although the heat given off rapidly diminishes after 1–2 hours.
Back in the good old days and still available now are basically giant Zippo lighters you stuff in your pocket. I could never get around the idea of having a fire going in my pocket so I only gave them as gifts and never used them myself.
Battery power is an option another option these days. Battery operated hand warmers are charged via USB and can also be used to charge your devices.
The last option is charcoal powered. I've never actually seen one of these in real life so I cannot comment on there efficacy but as I said before A fire burning in my pocket is contraindicated.
All these options are avalable at
Amazon,
Amazon UK &
Amazon CA to name a few online sources.
Here in Boston MA just about every Gas Station, corner store and even liquor stores sell that Hot Hands air-activated version
While I do use the air activated types what I really would like to try are the battery operated gloves and socks.