I've actually been researching just this for the past couple days because, like you, I think it'd be awesome to overland a Bronco. No companies sell a new camper fitted for the Bronco besides Phoenix Pop-Up
, which makes customs (very expensive).
Your second option would be buying a used one and sticking with it or refurbishing it. I have heard a few members saying that you'll find one on craigslist once in a while, but looking through some midwestern craigslists, I haven't found anything. Junk yards are also feasible options; I've seen a few trucks with popup campers (not Broncos though) at yards.
Your third, and probably easiest option for an actual popup, would be building your own. Here are a few resources to help. These European guys that made one for their bronco
, towards the bottom of this page
there are schematics and building pictures if you want a better look. Here's a few different links to help
you. Honestly, this or a homemade teardrop is what I will do in a couple years. It doesn't look hard, just time consuming.
Third option is being a redneck. Put a tent on the back and strap it down. Keep it in the back and potentially double your size. Not the best option but cheap and works.
Good luck! Here's an album of images about popup campers