RBI sells some, if you're looking for a way to buy them. I got one for each of my snakes in case of power outage so I can bag them in a cooler with room temp water. (Bigger risk for me is power outage in the summer in 100+ degree weather rather than winter)
If you're looking for sizing... I don't know. I just got them based on the size of their hides footprint and made sure it was at least that big.
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