find what?

i'd look at an online pet supply place.

It depends on the kind of dog how cold it gets and I am not sure if those heated mats are meant for outdoor use.

A typical dog doesn't belong outside for any extended period of time, unless a working dog of sorts and even then typically required to have proper shelter to provide dry food and water at all times that will not freeze. It may even be considering neglect if Animal Control was contacted.... I don't know your situation so not judging, just a bit curious as to why the dog needs a warm place outside. If it's too cold he/she can't just come rest inside?
They do make special "jackets" for working dogs that are well insulated. Can cost nearly $150 but they work well. My boxer won't go outside in the extreme cold without his, even just for a quick bathroom trip. he'll wait by the open door and scratch at the cabinet until We put it on him. They also make discs that you put in the microwave and use under bedding (I've used them for motherless newborn kittens) but not sure how long it would last outside, or if they have different sizes that would accommodate your dog.

In the house!

I agree with Amy!!