That's going to be hard to find. I'm not aware of anyone that makes one but I hope someone links it if it's out there. Most here build theirs but it's not a simple thing to build, lots of metal fab skills necessary to make it sturdy and safe. I would keep looking in salvage yards for a Chateau (Ford package with the folding rear bench) or a custom van with a fold down rear bench. Many of them had them. All you need is the frame so don't be put off by the condition of the seat. You would definitely want to refoam and recover an older custom van seat, or at least I would, and if you're OK with lap belts only that's pretty easy to add by just getting some universal ones and bolting them to the floor.
2010 Chevy AWD home build