I have a few questions on your situation.
1. So is your pop-top's fabric section totally gone, or there but in need of replacement?
I as that because (though I haven't replaced mine yet but it needs it!) it seems that most people that have done the replacement have used the old one as a pattern/template for the new.
Some people have made it themselves, others have mentioned upholstery shops, sail making places, all kinds of possibilities. I think it comes down to how much of the work do you want to or can you do yourself, and what services are available in your area.
Several people that chat here on the site have done this replacement, so it's very doable, and I'm sure they'll start chiming in soon!
Good luck, and once you do get going with it, post plenty of pics to show the rest of us how you did it, how it went, etc, K?