Target Walmart Bed Bath Beyond all carry them with the outdoor furniture but there a hard find in the best of times. A big patto or garden type store might be a better bet.

I get a catalog, probably because I bought the chairs and seats several years ago.
I think this is the site:
http://www.home…
the catalogs leave home fairly rapidly. I think the canvas can be acquired by itself -- I am going to need some new ones soon myself.
Props for fighting throw-away when it isn't needed.

try to find someone or a company that sells/repairs canvas tents/annexes etc they may be able to make new ones for you or at least steer you in the right direction for what you are after.

They are very easy to make. If you have a complete one, just use it as a pattern and start sewing. They only problem you may have is the needle. You will need a very heavy duty one.