Same situation this winter. Also had difficulty finding a pre-sewn fabric only store. My solution was to have a top made by a local upholsterer. I purchased Sunbrella material which I believe is the top material available. At least by price - almost $20/yd. Gave her my old bimini, the new material and $170. In a couple of weeks I had a new, perfectly fitted bimini. I am not sure if the Sunbrella is worth the cost though. You can find different bimini top material for half that price. My last (first) bimini from the Landau factory lasted 15 years. Which gives me an idea, contact your boat manufacturer and present them your problem.
Fair winds and following seas, Friz