Dealerships like Inland and Outofdoorsmart both offer parts that will fit. Problem is that they will likely be made of the modern alloy, and will not match your original skins, both in shine-ability and in durability. You might be able to find new-old-stock from one of these suppliers that is made of alloy that will match your trailer--you will have to kick some stones to find it.
My '73 is made of 6061 T6. I bought the same panel you are looking for from outofdoorsmart, but haven't been able to tell if it will polish up to match the rest of the trailer or not, because it has a factory clear plasticote on it that I so far have been unsuccessful in removing (another thing to ask about--coated or not). I have noticed that the alloy is much softer than the rest of the trailer--I already have a couple of dings in the new segment from road debris.
Your other option is to find a used panel from a 70's era trailer, but beware that AS was experimenting with different alloys throughout the 70's. There is a misconception that 70's trailers are all made of 2024 T3 Alclad, but I even confirmed with the factory that my trailer is made of 6061 T6.
Another option that may work just fine would be to carefully remove your segment, take it to someone with an English wheel, and some know-how, and have them work the dent out of it. I have an English wheel, and tried to do this, but was using too much force, too tight a diameter of wheel, and ended up changing the curve of the segment, so decided to go for a replacement. Colin Hyde said in a recent VAP podcast that they have had very good luck working the dents out of original segments on their English wheel. Problem is that shipping these segments is ridiculously expensive--I think I paid over $130 to get my segment shipped to me one way.