Fred's right, your *first* stop should be your local recycler who will without a doubt be able to give you the best price, especially if you find & pull them yourself. However, if you can't find what you want locally then consider a couple other options. Again, find something local but even if it's not to your liking you can usually have a local boat or upholstery shop recover them for a reasonable price and you'll be amazed at how good they look OR you can give Jeff's Bronco Graveyard a call. They don't list the interior rear panels in the catalog or website but considering just how many Bronco's must pass thru there annually I can't believe that there aren't some out there in the yard. Give them a shout and see what they can do for you, granted the panel cost and shipping won't be as cheap as finding them locally but it WILL be a whole lot cheaper than having a custom set fabbed by an interior shop.
http://broncograveyard.com/ There are other Bronco specific yards out there but #1) I don't keep track of them & #2) THEY don't support this site so I have even less reason to keep track of them & #3) I've dealt with Jeff's numerous times and always come away happy