Define what you are looking for.
A 12V heater for a dump load or heat? Or a heat exchanger for a solar liquid heater?
The only 12V heater that does Not use water from a heat source is the little 150W thing for $25. I don't believe they actually use 150W because the wires on all I ever saw were far too tiny, and I hear they do not last very long.
I expect a pair of head lights would make more heat.
http://www.princessauto.com/truck-trailer/truck-accessories/12v-heaters/8237190-12v-2-in-1-ceramic-h
eater-/-fan
The others are pretty much fancy enclosed heater cores (like from a car) with a built in fan, which require hot engine coolant to flow through and provide the heat, and 12V means it has a 12V fan. Could make one from a new automotive heater core and large (Huge) 12V muffin fan.
G-