Where can I buy a post office bike?

I really like the old work bikes, but theres not much special about the modern royal mail bikes.

Im sure you could build something of far higher standard very cheaply.

Maybe. I've not really been watching them on ebay or anything but have seen some (delivery bikes) that go for silly money. I guess some businesses (delis, restaurants etc...) use them as advertising nowadays (cheaper and better looking to chain up a delivery bike with a placard than pay for a bus stop advert)