If you want a brand new one, the only way you might be able to find one is to see if your local Ford dealership can check to see if there are any at any other dealerships (if they have a way to check that in their parts computer.)
Yup, like most parts for Cougars of that vintage, no-one has it and it is obsolete from Ford. Best bet would be to try Ebay, salvage yards, LKQ, etc. Last known MSRP was $273.47. That part number is just for the seat bottom cover.
I would imagine that not many were stockpiled to begin with,once those sold out,they would obsolete them.If dealerships didn't have any on hand beforehand,that would mean unobtainum.