This is the exact one that I used all of last Winter and never once had frozen water. It's a little pricey but I think well worth the investment to not have to worry about their water freezing.
http://www.amazon.com/Farm-Innovators-Poultry-HP-125-125-Watt/dp/B000HHQDPM
http://www.amazon.com/Farm-Innovators-Poultry-HP-125-125-Watt/dp/B000HHQDPM