You probably will never find anyone selling used canvas for a popup, especially for one 20 years old.
You'll have to source a new one from Bear Creek Canvas or CanvasReplacements.com
Dan-Retired California Firefighter/EMT
and Zoe the Wonder Dog(R.I.P.)
2016 PrimeTime TracerAIR 255, pushing a 2014 Ford F150 SCREW XTR 4x4 3.5 Ecoboost w/Max Tow Package
4pt Equal-i-zer WDH and 1828lbs of payload capacity