12 and 5 volts? Just off the top of my head, this looks like the perfect place to use an old computer power supply (ATX). I just got finished making my "bench power supply" from one. Supplies +12V at [lots of] A, same w/5V. Well regulated, too. Probably available for free. Tons of web pages out there showing how to make the conversion (really simple: basically connect one signal [PWR_ON] to ground).
Oh, and contrary to what these DIY instructions all say, I didn't need to add a dummy load resistor to make it work.