I just bought one on eBay last night for mine. I want something light weight that I can cram in a stuff sack in just a few minutes, and not have to fold and store a heavier cover. I went a little over size, bought a cover for 20 foot boat with 100 inch beam for $25.90 and free shipping. For another four dollars, I bought 20 of those plastic clips that you can attach anywhere around the edge for your tie-downs. I am 100 yards from my neighborhood ramp, so I'm not concerned about trailering, only leaves and junk in the boat When it is parked under the trees.
For about $150 on eBay, you can buy one which says that it is made for the Parker 1801. That will probably be my route for a winter cover.