I bought the snaps from a boat cover shop, come in three parts, first is this one, the part that screws onto the body. 
Then comes the two part "hat", you have to buy a special tool to put this part together, you punch a small hole in the material, and put the cap through the material and the hole in the female end, then squeeze it
The material is called Sunbrella. 10 year guarantee against fade/rot, and that's with 24/7 exposure to the elements. Waterproof too.
