Where do I get a right hand wheel vehicle to use for postal service?

You might find some surplus post office vehicles on some of the internet auctions sites like Govdeals or Govliquidation, or try your local governments auctions. I personally don't really like those internet auction sites, you'll pay way too much with all their added-on fees, if you go to your cities administration's center or PD you can ask when and where they might hold such auctions. For my city it's usually every 6 months, when they unload the impounded cars and old city vehicles.

There is a website that specializes in RHD postal vehicles. ---- https://www.postalclassified.com/