Neil -- I got mine when I bought the camper since I knew I would like some extra room. While I was less than thrilled with the walk through I got on the use of the camper from these folks, I could not beat their price so I drove 425 miles to Toledo, Ohio to make the purchase. I'm a serious price shopper, so that says something.
I have no idea where you live, but they draw customers from all over, so it's worth a call to talk to them: Labadie RV
Let me know if you have other questions. I was so, so impressed with the screen room I can't wait to use it again.