I would look to Cabela’s, Overtons, Fleet Farm.
Long story short. I used to deal with same basic boat, ’83 Lund Pro Anlger, came with a very nice custom cover, fit real tight, built a support structure out of PVC pipe, did not glue all the joints so I can take it apart and store for the Summer. My nephews have this boat now, they are on the thirdcover for it. Their cover is loose and baggy, I would think they could have found a better fit, but they are still using the same PVC frame I made back in the late ’80’s. I keep telling them to modify or start over with structure design, they just stack bricks underneath PVC frame to lift it more. I always put one of those silver tarps over the top of the canvas top for the Winter, made it much more waterproof. I was always able to brush the snow off of it during the Winter, so weight build up was never an issue. Biggest thing is make sure you do not have low spots where snow/water collects.
OK, so story not so short. Bottom line, you should be able to find a off the shelf tarp and make a support system of some sort.