Yeah I took my little girl to the vet and I was afraid she'd get cold (drove 30 miles to get to one of my coworker's vets who said that this guy was awesome - she has 2 snakes and a monitor). I just got one of my hand warmers that I like to take hiking on cold trips and wrapped it up in a pair of socks. It didn't get too hot and she just curled around it

The kind I get are the small kind (I think the "feet warmers" get too hot) from REI for about $1.50! But I'm sure they're in all sorts of other places too.