i think i do remember running across one of your fabrication threads before..
5v is pretty easy to get from a car.
if you are doing a 'standard' carpc with atx-compliant power supplies(m-series psu's, dsatx, etc), there is a 5v wire that you only need to tap into on the 4pin flat harness.
if you are using a laptop, i believe the carnetix series psus do have a 5v output, or have a add-on module for 5v that would work for that.
and in the worst case-- you need to buy one of those usb cigarette lighter adapters. those put out 5v, but you might need to modify certain parts to get everything connected correctly.