As shown in the pictures, on both sides of the car between the rear wheels and the doors, I need to cut out and repair the rust. I know I can buy outer skin patch panels for the exterior sheet metal but my concern is the structural members that are rotted out. As you can see one of them is completely rusted and has been cut out. Are these pieces available on the market or do I need to just fab something up to substitute for them? If so what are they called and where do I get them? I know I will need to cut some more of the metal out and have concerns about access to weld new members in. Any suggestions is what I am looking for.

Check out this link...AMD sheetmetal Not sure what is all included with the full 1/4's. Someone else will chime in or you may have to call them.

The part that's cut out is the outer rocker. You probably have rot in the bottom of the outer wheelwell and the door jamb to go along with it. Once you get past the leading edge of the quarter panel, that inside piece becomes the outside.

I bought a Duster that came with about a 2' long section of rocker, so somebody's making repair parts for it.

This is my '71 Charger, but the inner structure should be the same. Thought you might like to see what's underneath the skin. I edited the photo so the orientation would be correct but the website software flipped it sideways.

Just click on the image and you can rotate the image 90 degrees clockwise at a time until it's in the correct view or orientation.

When you see it on the outside like this, it's EVERYWHERE. What do you suppose the insides of the frame rails look like? Rusty cars from rusty climates are a structural nightmare.

That pic from 71Charger explains to me the construction . I now realize it is the rear section of the rocker that is rotted out. It looked like there was a joint in the rocker where I cut it out but now see it was one continuous member. I restored this Texas car back in 2006 and it was pretty much rust free, except for the trunk where water had been sitting and these are the only 2 areas that have rusted since then. The frame rails and everything else have not deteriorated. The cause of the rot is my fault. Someone (me) sandblasted the trunk back in 2006 and that's what created this problem. Sand had filled the areas and any water that got down there, probably from washing the car and any leakage around the rear window seals was retained by the sand (garnet blasting media) and caused the rust. The rockers toward the front are still solid. I'm thinking about buying one rocker, cutting it in half and doing a splice job on the rockers. This is not a show car and I don't plan to sell it. My son and I do all our own work, so taking to a shop for a professional to do the job is not part of the equation.
Any feedback is appreciated and taken without offense,

Here are a couple more shots showing the inner structure.

Good pics, thank you

Buy a clean donor car. I would think a 4door car would have the same parts. Chances are you will need something else, is very likely.

This is what mine looked like.

After patching

Thanks for the help gentlemen, I started on the drivers side removing the rusty metal and started cutting patches I will weld in. The damage really is not as bad as it looks.
Thanks again.