Question on pickup trucks with refrigerator box units -?

the small company I work for is trying to find a reasonable used small reefer refrigerator unit to bolt on the front of one of our pickups - the truck already has the 8' white slide-in aluminum insulated box, but just no cooler unit on it. I have searched every where I know of from google to various classifieds & cannot find anyplace that sells pre-owned small reefer units for pickups. Does anyone have some knowledge/experience in where to buy these units? Thanks alot.