When I was looking for a J truck, I saw a few that had regular step bumpers installed on them. Another one had what looked like the rear bumper off of a Ford F150. You could probably get an aftermarket step bumper with generic brackets and make it work, but of course it's not going to look as good as a factory one would, but it'll serve the purpose.
Other than a custom bumper from BJ's, you can scour sites like Ebay and Craigslist for a factory one.
Also for a factory bumper, here are some other places to check.
Montana Overland, they specialize in used FSJ parts, and might be able to help you.
The parts section at JeepTruck.com.
And certainly don't forget the parts section here.
Here's a picture of a J20 with a step bumper to get an idea of what it would look like, it doesn't really look that bad in my opinion.