The mill I used to work for had huge plywood crates shipped from Germany for parts. Nicest plywood I have ever seen.
1" - 9 layers (ours was 5-7 depending on manufacture), no imperfections, nice coloration and face matching veneers. One fellow begged for the crates and made kitchen cabinet doors out of them. Nicer than you can get locally - and that was crate material for them! Beautiful reddish brown, much like teak.
I'd like to see what their good stuff is!.
If you can use 48" stuff, try trucking yards and large warehouse operations - preferably that sell heavy stuff. They get the hardwood pallets as they don't break. Produce and groceries get the light ones you step through the slats as they are so flimsy. Remember to ask permission. Be polite and be neat. Leave a mess and never get to come back. If you get permission, a box of donuts to the shipper may get a return trip.
If you need longer try, windshield shops. Theirs are usually 6' or so, but are pine.