I just had to call Scamp
. They use all Alum rivets with washers and to fancy it up he said acorn nuts. So yes they are just pop rivets. Buy them at your local store people.
The washer would be used to expand the area that it holds on the inside. It also can be use if the rivet is too long. Scamp
sells nothing special on the rivets. So buy the cheapest you can find. I also noted the size they use on their web site if your not sure. Remember to always use the Butly tape and or seal and make sure the rivet goes in easy and not tight. A rivet hole should never be a tight hole.