http://www.meci.com/Catalog/Category/351a1e14-68ea-4bf1-b992-9029fc6b6669
I have never purchased from these people, so I do not endorse them. I just noticed that the prices looked good.
Remember the following formulas Power (watts) = Volts X Amps (in a DC circuit)
Volts divided by Amps = Ohms (resistance)
For example, the first heating element rated 4500 watts at 208Vac can handle only 21.6 amps flowing through it. It has a resistance of 9.6 ohms, and at 12 Vdc it will only draw 1.3 amps for a total wattage of 15 watts when used with a 12Vdc system.
Therefore, these immersion heaters would not work, but if you could generate high voltage DC with a Industrial PM motor, these would work well.