Frequently, when a designer sells a set of plans for a boat they include a license authorizing the construction of one boat. Since full-sized plans for s&g boats are paper templates which are attached to the wood panels and cut through, a lot of times they are used up in the building process. For those reasons you may have difficulty in finding second-hand plans.

Consider a few other options: Search for a design which suits your paddling skills and size, and buy that design, paying the going rate. Search for inexpensive plans and pay what you can afford. Usually this means more generic design. Clark Craft www.clarkcraft.com is one supplier of plans for older designs at nominal prices. Buy a book which has kayak designs in it. 'The Kayak Shop' and 'The New Kayak Shop' by Chris Kulczycki both have reduced-size designs which you'll need to draw out to full size. You can do this on paper or directly on the plywood. There are a few free designs available on the internet which are simply lists of offsets. You graph these numbers and connect the points to create the full-sized shapes you need. It takes enough time that many people opt to buy the full-sized plans.

There are some other options. You could look for designs for Skin-on-Frame boats, and build them in plywood instead of fabric. There are a lot of books with designs for SOF kayaks. Many have no license fees. You build an outline of the boat using inexpensive wood (pine, fir, spruce, etc.) for the chine strips, tacked to plywood forms. On this framework you place sheets of plywood. They'll bend easily. Trace the shape along the chines , cut the panel, and tack or staple it to the chines .Do the adjoining panels, then stitch or wire the panels together. Remove the interior framework and you have an S&G boat.
Hope this helps. Good luck with your project.
PGJ