At one point I did consider getting a pet heating pad or bed but I was worried about the cords and what if the puppy peed on the pad and got electrocuted...
So I bought 3 snugglesafe heating disks (one each for the two kitties and one for the puppy).
I feel these are safer than the heating pads and water bottles (you don't want the dog to chew the water bottle and get scalded).
Here is the link:
http://www.snugglesafe.co.uk/
It's sold at Petsmart, but I ordered mine online.
I haven't used mine yet since I only got it a few days ago and it hasn't been too cold. If you do a search for "snugglesafe", I think some members here have used it.