Heating source for a feral cat house
I live in New York and two years ago we "adopted" a couple of feral cats.
We had them fixed, feed them several times each day, play with them to an extent, and bought them cat houses which they use each night.
Can anyone recommend a heating source for the cat houses.
I was wondering if someone sold an insulated heating blanket or pad for them to sleep on but cannot find any.
Any suggestions? Thanks.