I'm not quite sure just which panel you mean or both? The left side, behind the wheel, I believe can be had, Worst comes to worst, Try Howells sheet metal. If you mean the rear splash apron (Across the back of the car) I think this panel can be bought used. I don't believe any one is making them as they're only used on Town sedans. I needed one for my 30 Briggs Town Sedan. That year, I found three of them at Hershey. The year before I found two, and they weren't cheap. Take the rear seat out and (I think) you'll see how it's connected to the body. The side, inner fender panel can be made and welded in with out too much trouble. Make sure the patch panel big enough to cover all the rusty metal. Don't leave any "Lacey" areas.
Terry