Try two sources for marine plywood. Btw, any lumber yard (even Menards) will order it, might not be exactly what you want though.
Try Glen L, it’s a home built boat place that has all sorts of links for building materials, just do a web search for “Glen L” without the quotes.
Boatbuilding is Zen
This is a framed poem that hanges in our home.
House are but badly built boats, so firmly aground that you cannot think of moving them.
They are definitely inferior things, belonging to the vegetable, not the animal world, rooted and stationary,incapable of gay transition.
The desire to build a house is the tired wish of a man content thenceforward with a single anchorage. The desire to build a boat is the desire of youth, unwilling yet to accept the idea of a final resting place.
It is for that reason, perhaps, that when it comes, the desire to build a boat is one of those that cannot be resisted. It begins as a little cloud on a serene horizon. It ends covering the whole sky, so that you can think of nothing else.
You must build to regain your freedom.