If you go to E-bay and enter "Sunfish Sailboat", there are several types of Sunfish covers for sale at decent "Buy it Now" prices. However, in my book, the Sailor's Taylor makes the best heavy duty Sunfish cover. Google "Sailor's Taylor" and you can find their website.
I just bought a new ST mast up cover - the old one lasted about 10 years outside 5 months of the year, and I sold the old one for $25.
Check out Fabricraft Covers, too - email@example.com - Talk to Craig Thayer. They made a wonderful cover for me. It's really well made. Sailors Tailor is good stuff too and I have one of theirs on my Snipe. Can't go wrong with either one in my opinion!