I think there are two broad classes of "plywood kayaks", the rough kind made
in school shops (etc.) from hardware store plywood, and the more modern
kinds made with thin specialized plywood and sheathed in fiberglass.

The two main sources for premium kits a

http://www.pygmyboats.com/
http://www.clcboats.com/

The best resource I've seen for kayak building in general is this web-board:

http://www.kayakforum.com/KayakBuilding/index.shtml

It's sponsered by a builder of stip-built boats, which are very beautiful
themselves:

http://www.guillemot-kayaks.com/

HTH