They do exist and I think horse.com or valleyvet sells them. However, it's a whole tank unit that you have to buy and it's about $300 or something.
As an alternative, you can buy a panel and a deep cycle battery, build a box to protect battery and wires and wire a heater to it -- I've done this with a car stereo that we used for endangered species work (long story).
I have not found anything after extensive searching, but there are some things you can do to help keep water from freezing. First, use a stall mat or other insulating material under the water troughs to keep them off the ground. Heat transfer into the earth is a big loss. Build a wood frame around the water troughs, use attached hard insulation to completely surround it. Leave only a small portion of the water trough accessible during the day, and cover at night (or when no horses are in field/paddock). Information I've seen indicates this will keep water freezing in winter in Wyoming (slightly hard to believe, but I'll suspend disbelief).
If you want to take it to the next level, you use a house radiator with copper tubing that loops into the water trough (and of course out of harm's way). There is some change in temperature of the water in the loop that causes the water to circulate through the system, and the absorbed heat of the radiator will help keep the water from freezing. At least I think that's the way the engineer who wants to build me some of these explained it....