To answer the OP's question: A real lumber yard (not home depot/lowes), will have or be able to cut for you pretty much whatever size you want. If you ask a local cabinet wright or furniture maker they can give you a good lumberyard recommendation.
And now to add my part to the plywood/lumber discussion: Plywood may be stronger and perhaps more stable than wood... but an exposed white oak transom with a nice coating of resin or something for waterproofing looks a lot nicer than plywood, and can still be built much stronger than required. So, that's the way I went with my 1432. Haven't regretted it.