Unless you are building a mini tubbed 1st gen Camaro you are probably out of luck. Of course I haven't been looking for inner fenders for a '73 Charger. Do a google. You may find a bone yard some where that may have them. Of course, when you remove the outers, it would be a lot simpler to work on the inners if you had to rebuild the weld seams or the mating edges to the quarters.
You may even consider using junk yard stuff if you can find a later model car with a similar fender shape. Cutting up late model parts to fit into your Charger would be one way to go.
Mark