There's a variety of sites. Google "Postal Jeep" and "AM General" and "DJ5" and you'll get a variety of hits. One site specialized in listing items for sale.
Why do you want one? The last one made was in 1985. The last ones used by USPS weren't sold - they were destroyed. The odds of finding one worth buying are pretty slim.
If you just want the look, you can maybe find the body of one in a boneyard and bolt it to a pre-1976 CJ frame. It's basically a bolt-on. If you want RHD, you'd have to rework the steering setup, but that's not hard either. Hard part will be finding the Ross worm-and-roller steering gearbox.
The Postal Jeep, as it was used, wasn't a terribly user-friendly item. The Kaiser models used a Chevy II engine and Powerglide two-speeds. The first AMC/AM General ones used the 232 six (very heavy) forward of the front axle and a Borg-Warner "Mickey Mouse" auto tranny. That was an item AMC had warehoused but had so much trouble with, it wouldn't install them in the Gremlins which was the plan.
The first sixes used the same 6:1 rear end as the fours and the military jeeps did. Gas mileage of 10 mpg and a top speed, RPM limited, of 60. And, tippy? You better believe it. I nearly rolled one in traffic - with a quick lane change.
Later models used VW fours and finally AMC fours. Torqueflite became the standard and they finally, after the 1979 fuel crisis, dropped the rear-end ratio. I don't know what the fuel mileage in those models were, but it couldn't have been high.
You're better off putting the box-body on a CJ5. Trust me; I owned three Postal rigs.