Where can I get a replacement shaft for a Wilson Fat Shaft 9 iron
Wilson made these for years, so you might be able to find a Fat Shaft 9 Iron on Ebay even if it’s a different model than yours and part it out. Truth is these were parallel tip so any Fat Shaft iron should do. I would think you could buy, for example, a 6 Iron and tip trim it to length.
The only other idea would be to contact Wilson directly.