Kind of an aside, but we get our dc-dc converters from here:
http://www.current-logic.com/dcdc_converter_car.php
Unfortunately, I don't see one that fits your immediate needs.
We typically pick up a Mag light from Lowes or Home Depot and mod it to use a 12:5v (or 12:3.3v) stepdown converter. Then, use a SPIKE as a switch.
(I like Bruceb's and apalrd's approaches as well )
Thanks,
Eric