I don't know where they get them, but when I had new cushions made, they came with Velcro strips sewed on the bottom of the cushions that tend to slip around, and had plastic Velcro 'buttons' that are attached via a screw. I had the cushions made by U.S. Yacht Cushions down in St. Petersburg, FL, so you might want to contact them and ask.. they are very friendly and helpful and I'm sure they could either supply you with some or point you in the right direction.
Cheers,
Brad
PS: These were for interior cushions, but I see no reason why they would not work for cockpit cushions as well. I've had cockpit cushions like already mentioned; with the non-slip bottom covering, and while they work fine in most conditions, they still will move around and could even fly out of the boat (ask me how I know!) when conditions get a bit rough. YMMV
