I know here in Knoxville there are a couple of plywood wholesalers that carry 1" plywood for specality purposes. What I saw when we toured them was for the boatbuilding and safety cabinet industries. Awful expensive, since it has good plies all the way through. Maybe there is a decking grade that would be more affordable? I think maybe a local lumber dealer could order it for you. My idea of a substitute would be 3/4' plywood and some spacers or glue and screw two half inch sheathing sheets together, or glue and screw a sheet of 1/4 inch sheathing to a sheet of 3/4.
I have seen 1" baltic birch at a local woodworking shop, but it comes in 5' square sheets.
I would be very worried about the glue in a hardwood plywood or in imported panels. One good thing about the southern pine sheathing is they use a glue that is very, very waterproof, even in the interior rated panels.
I expect if you look into the boat building suppliers you will find some wonderfully waterpoof 1" plywood.