NOT "hand warmers"!! They get way too hot and they only stay hot for a short period of time...neither of which is any good for the snakes.
You should go to ShipYourReptiles or any other shipping supplier and order real Heat Packs. They come in different measurements and you want 40 Hour Heat Packs, minimum. They also come in 60 hour and 72 hour versions. Any of those will work. They should be ordered in advance to have on hand when an emergency hits. You can't find them at your local pet store or Wal Mart.