Replacement seats
I would first see if the outdoor co. has a replacement, which you probably already checked and found no replacement.
There are companies on the internet that sell replacement seats for tree stands. Key in -replacement seats tree stand - on your computer search. It's a matter of searching
I've replaced seats on a tree stand by buying from a local fabric center. They even had foam that was cut to size. It pays to know someone who uses a sewing machine.