I seriously doubt that you will find the molding anywhere but the manufacture. I do not remember steel doors on residential 30 years ago. My guess the doors are of a much later manufacture.
I bought some steel French doors from Lowes. Same basic construction as you describe. I decided since I did not want to cut the block wall for the doggie door. I would take out one of the vent windows next to the door, the width was perfect. I located the manufacture and asked if they would custom build a side light for me... NO was the answer. I asked if they would sell me a side light window.. "HELL NO was the answer. We are in the business to make door sets we do not sell parts and pieces". I asked to speak to the management. They hung up on me.
Good luck, might be time to consider a front door replacement.