It's very thin plywood variously called hobby plywood, craft plywood, etc. That kind of stuff is available in hobby shops and the web. The thickness varies, depending on the scale of the model you're building. The thinnest I've seen is nominally 1/32" (0.8 mm for the rest of the world that hasn't landed people on the Moon yet). It cuts like a very tough cardboard. A razor knife and straight edge or even a paper guillotine makes it easy to work with. CLC uses a laser cutter. Thicker pieces start behaving more like wood.

It's also fairly pricey, on the order of $30 or more for 2 square feet. Other sizes exist, of course.

I have not bought any mail order. only from local hobby shops where I've had to pick carefully to avoid warped pieces and pieces with defects. I asked CLC if they would consider selling just the wood since theirs was such good quality compared to what I've seen in hobby shops and was told that the biggest problem they have is sourcing wood that meets the quality specifications for their model kits. As a result they were having difficulty finding enough for their own use, let alone to resell.

If the plywood is too much money or trouble, you can substitute other materials. The cardboard from cereal boxes when coated in epoxy, varnish or paint becomes stiff and waterproof enough to make a model that can actually be put in the water. Depending on the tools in your workshop you can also shave DYI store dimensional lumber, popsicle sticks, etc. to the correct size and shape to build boat models. If you really want to see what's possible look at the book Tiffy’s Workshop – Marine Models From Scratch by Graham Castle. Lots of good stuff in there about techniques and materials.

Have fun,

Laszlo

   Wow, Laszlo, thanks so much for all that helpful info. The cereal box suggestion resonated with me. My first scale model boat was built using posterboard that I purchased at an office supply store. I laminated two pieces with contact cement to simulate the thicker plywood of the hull bottom panel.

I always appreciate reading your pespective on this forum and appreciate the tremendous contribution you and other regulars make.