The Solid Fuel Pocket Hand Warmer has been discontinued. If you're looking for something new, check out the best survival gear for 2020.
Historic Range: $4.99
I have used solid fuel handwarmers for 30 years. They put out more heat than the chemical handwarmers and each fuel stick lasts for hours. If you want even more heat you can light both ends of the fuel stick.
The down sides are that you need to light them which can be touble if you don't have shelter and they are a bit too bulky to slip into gloves.