You can get a cover designed specifically for your hull
or you can buy a canvas tarp a couple feet longer and wider than your hull.
Both are for sale on line, some quite inexpensive.
Remember to support the center over the hull so water runs off.
I've seen some interesting boat shaped mosquito ponds.
I went with the Harbor Freight tarp with center supports, much cheaper. I bought a custom fit cover for my 20ft and even setting up several supports I had water issues and tore the cover. I finally went with the 10ft x 20ft car cover / car port. Covers the boat very well and you can hang a light inside and work at night when the wife wants to watch some crappy period movie (Jane Austin or some garbage like that).
Yes, I use a car cover from costco,they have 4 different sizes, try to estimate the size of the boat, to the sizes of cars listed on the side of the box, works great, 2years old, spray it about every 6mo. with can of silicone that you would use to waterproof tents, and also support in middle to keep water from puddling. Ed
If you decide to go the tarp route and have trouble finding a size you like or want, you can get a custom cut tarp for just a couple bucks more than standard sizes. here is where I have be buying my tarps for boat covers for years and with my lastest boat I found none of the standard sizes made a decent fit. Good prices and quality heavy duty tarp that will last in the outdoors. A couple PVC braces or even a simple pvc standoff in the seat posts will provide a tenting effect to keep the boat dry.