As mentioned, there are various sources.
A good place to start is your local hobby, or craft store, for styrene sheeting (scribed is easier, or just plain so you can scribe your own deck), and/or balsa or basswood decking. You can use mechanical pencils on the latter, to scribe the boards into the wood, with the aid of a ruler.
For masts and booms, you can just use wooden dowels, or barbecue skewers, cut and sanded to shape.
For specialist naval fittings, and cannons, you can either make your own, or purchase them from miniatures manufacturers, or those selling wooden ship kits. The companies selling the latter do vessels in 1/4 and 1/8 scale (1/48th and 1/96th scales, respectively), as well as others, e.g. 1/64th scale. Those equate to roughly 25mm/28mm and 15mm/18mm scales for minis.
Some people get parts from plastic sailing ship models as well, and use those too.
Cardboard and/or posterboard is a good material for making the sides and stern of the vessels, and can be pinned to a wooden deck, while the white/wood glue dries it all together.
Of course, you can also use sheet styrene for the ship's sides as well, but it is a little harder to work with than the paper products.