here are a few places that deal with boatbuilding woods...the ones that have PW sell plywood. You could also try a contractors lumberyard and see if they can get you Plyfloor underlayment in 4x8 sheets. It is baltic birch with 5 plies, no voids and exterior glue. Pretty good stuff really and about as rot resistant as occume...which is to say not very. It is a bit heavy...around the same as hydrotek meranti plywood.
Caledonia - Kettle Moraine Hardwoods, 195 S 27th, 414/835 9212 - L (4-00)
Green Lake - Norton Boat Works, 535 Commercial Av, 54941 414/294 6813 - L, PW
Hartford - Kettle Moraine Hardwoods, 5261 Aurora Rd, 53027, 414/644 8119 - L (native & mahog.)(4-00)
Madison - McCormick Lumber, 3156 Milwaukee St, 53714 608/244 4741 - L, PW
Plum City - Grange Hall Hardwoods, Inc., W2326 US Hwy 10, 54761, 715-647-4544 - L