Really depends on what you want it for also. I bought 15 sheets of 3/8" waterproof marine plywood (7ply) in mahogany and I think I paid $73 a sheet, something like $1200 delivered and it came from Maine to KY. Not a bad deal. I still have 6 whole sheets left (I think) which should be plenty to finish the job. I've been using mahogany and oak from the local store to make the trim. The sheets are very flat and stable. Using Helman's Spar Urethane to finish.Jim
You need a proper mill / lumberyard. If you let us know what part of the country you're trying to find this I'll look through some of my old vendor lists from when I had a wood shop and see what I can find.Another place to look will also be a plywood supply house. This will be unlike a typical box store and will stock or have access to countless types of sheet goods including an endless variety of furniture-grade hardwood veneered plywood. Great way to build cabinet carcasses for visible areas, especially if you'll be putting solid hardware facings in front of it.