You've actually got two pieces rotting away in that particular spot. The outer panel where it folds in to be spot welded to the inner structural panel. Point being... you can get outer patch or replacement panels, but I don't think you can get the internal, structural panel piece out of a box. I could be wrong though. :shrug
Your best bet would be finding an old 78/79 in a junkyard or that someone is parting out a wrecked one and get a that chunk cut out for you to patch/replace. Obviously, that's going to be a Bronco specific piece with only 2 years to choose from, so you can call around, rather than driving to every JY in your immediate area. Should be fairly obvious to any yard rat with 1/2 a clue.
Or you could just get some scrap metal and bend a piece to cover the entire rotted area, cut out what's bad and trim up your own patch. :shrug
You should drop into your User Control Panel and fill out the Bronco information area so we can all see what year, model and other info... so you can ask without explaining it over and over again. You can see my info under my profile name. Very useful.
What appears to be rust in the photo are areas where I removed seam sealed and exposed bare metal. There is a small hole in the photo which I,m not to concerned about. I do plan on fixing it though, easy repair.
I do have a patch fabbed up to use if nothing else comes up.
You will not find much useable in yards around here. All the knuckleheads haul everything in for scrap. What did not go for scrap is usually rusted to death.
I,ve been attempting to find a way to buy patches from desert state trucks. Thats not going well,