I bought mine here in Louisville at Boland Malony Lumber. I would think any "real" lumber yard should have it (not Lowe's/Home Depot). While I was buying it, the "wood guy" exlained to me that it is the glue that is used in marine grade plywood that makes it different --- the glue is water proof and that's why it works and lasts so long. I know others have mentioned pressure treated, but there's a reason they make marine grade. Yes, it is more expensive, but it's made for the job.

Good luck with your project.