There are a bunch of stock ones on eBay. I can't imagine they are all genuine Nissan. Where they come from, who knows.
As far as aftermarket offroad/heavy duty style bumpers, Shrock makes one, so does KMA. A custom rear bumper should be a quick and easy job for any competent fab shop if neither of those two fit your needs.
Another thing from Hardbodys I know is interchangeable is the tailgate, and the beds are the same size. They are WAY more similar than most may think. A first gen Frontier is more closely related to a Hardbody than a second gen Frontier, they have absolutely nothing in common other than a model name.