Hello Lee,
Reverse polarity conversters are one of those things that whilst being a valuable accessoriy for caravanners, they cannot actually be purchased.
You can buy all the component parts, but you cannot purchase it ready made up.
The reason is that all electrical appliances and extensions must be manufactured to comply with the wiring regulations. These stipulate that the polarity of any systems must conform the wiring standards. A crossed over assembly, would fail the test becasue the phase and neutral wires are crossed!