They are easy to make by gluing three layers of standard 17 or 18 mm plywood together.
Thomas Raun-Petersen stocks them ready for use. Thomas is the secretary of the Scandinavian Wayfarer Association and he also runs a little “Wayfarer special parts” shop from his garage. He may be participating in the Worlds (Weymouth) next year and if he shows up, I expect him to be there with his parts van. If you can’t wait that long, try his web shop: http://www.wt.dk/. shipping costs may be prohibitive though (weight).
BTW, Thomas speak English very well and always finds the time to help a fellow Wayfarer sailor.