Are you looking for the rear portion of the frame rail? If so, any Mustang supply shop sells that section of frame rail. It's not hard to replace. It's very common for them to rust out where they have the doubler on the inside near the mount for the rear spring hanger eye. I had to replace both of mine.
Actually I asked this question incorrectly. What I need is the bracket that the rear leaf springs slide into and the bracket is welded on to the side of the rear rail toward the front of the car NOT the shackle end.
Sounds like you're talking about the rear torque box area. You have to buy the entire torque box to get what you're after. Here's the best picture I could find of a rear torque box. That's the frame rail you see heading out the left side of the picture. This is looking at the bottom side of the torque box.