Where to find replacement popup canvas

Try searching for awning and tent makers in your area. With your old ones for a pattern, they should be able to make replacements. Years ago my Uncle ran a marina and quickly realized that there was a demand for the soft, canvas, upholstery products as well as mechanical. My Aunt, always handy with a needle and thread, soon had a thriving business. Different options, especially if your RV manufacturer doesn’t offer replacements or out of business.
Another option would be to ask an RV dealer where they get there repairs or replacements.

Ken