I would think you could put one together pretty easy/cheap, if you have some sort of a water tank.. and use an "instant hot water" deal from home depot (small, people usually put them under bar sinks).
edit: Never seen these in DC.. but power inverters are pretty cheap these days.
Instant heat is applied to the water as it passes through. No storage tank to deplete! Constant water in, constant HOT water OUT! Battery ignition starts the flame ONLY when water is running through the unit. Delivers 1 gallon of hot water per minute, with adjustable temperature settings from 80ºF to 150ºF. No overheating. Includes shower nozzle with on/off control, hose and regulator for attaching to your standard LP tank and folding handles for easy carry. Standard LP tank will provide up to 18 hours of use depending on temperature selected. 2 ”D” batteries for ignition (not included). For outdoor use only. Measures 12” x 15” x 4”.
http://www.majorsurplus.com/Tankless-Portable-Water-Heater-P1691C66.aspx