i working electronic component shop before, now still working of electronic relate job, already use and look many model of the solder gun or pen, but haven't c got 12Vdc one. I recommend you using "GAS" (same with lighter's gas) solder pen. Light, small, easy to portable, and can replace lighter .
I too have not seen a 12v soldering iron in pasar road, only the portable soldering iron that uses lighter gas only. I have seen a Tamiya Soldering Iron that uses 7.2V or 12V batteries to solder, but the wattage is quite low to solder heavy gauge wires.
i found the solder gun..but 60 buck..for only 12W ...haha..so end up leave the shop directly..btw, anyone can help me to find some disco strobe light, i wan the cheapest the better...