Try he http://www.marine-plywood.us/
I think they're 10 hours from you, but I bet it's worth the drive.
I've been to the shop several times, great service. I haven't bought
any marine ply from them yet, but I wouldn't hesitate when the time
comes. The stuff on hand looked really good. Call/email and ask
what's in stock. There should be plenty as there are always guys
redoing their Lymans this time of year.
Good luck,
Sam (no affiliation yada,yada,yada)

I went there for the first time a couple of weeks ago. Good people.
I'll be going there on Saturday to buy the lumber for my dining table.

The neatest thing about the shop is the table saw setup. They
straightline wood for a small fee, and the way they do it is with a
power feed table saw. To line up the cut, there's an incandescent
light fixture with a string tied across it hanging from the ceiling of
the shop over the saw. The shadow that the string casts is how they
line up the board for straightlining. Pure freakin' genius.

-Phil Crow