Some Manufacturers import RHD versions of their vehicles specifically for fleets like the post office. I know Subaru and Jeep offer them here. As for retrofitting a car, most of the dedicated-duty home-brew conversions seem to involve running a chain over to the passenger side. You could watch eBay and see if maybe, just maybe, a B3 or B4 RHD turns up. Odds of it being diesel are astronomical but you could use it as a parts car for your conversion.