HiTech
You seem pretty uptight with not finding this 6" damper.

I'd guess if there is a real demand, they would be available. Further am guessing there is not a real huge demand, but it may be good enough to start a small business making them (hint).
With a website that comes up with google search, seems it could be a worthwhile side business, even if you have to contract with a casting co. to make them for you. Would be nice to see them with "made in USA" instead of China.