Walgreens sells little hand warmer type heat packs that last 10 hours. They are $.99 for 2.

But, if you have a longer outage, here is a link for 60 and 40 hour heat packs. They are pretty cheap. I'm gonna nab some soon for shipping my beardies.

http://cvexotics.safewebshop.com/60_...eat_packs.html

At the bottom of the page there is a link for the 40 hour packs.

And as for using them, I'd say to wrap them in an old towel, and the snakes will probably curl up around them. Probably best to put them in one of their hides.