That panel that you removed is really not in bad shape. It looks like it could be wire brushed, paint removed and use muriatic acid to dissolve the rust. Even if you have to have bits of it cut out and welded in you are typically better off with final fit, instead of finding an aftermarket panel.
Underneath looks a little holy; that's a problem area because water is allowed to drain through it. You'll need to remove the back glass like has been said. Remove the trunk lid, clean out the rubber drains, get new parts, wire brush all the rust out and weld in new to make it solid and strong. Since it is hidden, your repair doesn't have to look pretty, and you can use POR15 or similar rust converter for the lightly damaged areas.