I don't believe the big box stores like HD carry the marine grade ply. Sorry you are not closer, as I have some left overs from my project. I got mine from a local wood supplier, but they had to order it. If the rest of your sub-floor is not marine grade, I would just get the best ply you can find locally, and make sure you seal it really well. Most sub-floors are 3/4in., but I have read about some 5/8 as well. You should be able to use as smaller mesh, as long as you get good air-flow. But a smaller mesh will also clog with debris faster, so you will have to keep an eye on it.