Check out McMaster Carr. They are a great resource for parts like rivets. They don't carry marine gear but for basics like fastners they are a great resource

http://www.mcmaster.com/

go to page 3114 of the catalog. I really like the high-strength 18-8 stainless rivets. The 1/4 inch rivets are great for re-riveting Hobie 16 castings and crossbars.