Bill!
Check with your local,
independent cabinet & hardwood suppliers ... I live in a town of less than 100K, and I have two hardwood companies and three independent lumberyards who can
all get plywood in various thicknesses in sizes 4x10, 4x12, 5x5, 5x9, 5x10, and I can even get 5x12 from two of them. The bigger stuff usually isn't stocked, but none of these guys have ever charged me anything for having it brought in on their next truckload ...

...!
Since you are already here, take a good look around at the various build threads ... you'll find many, many, many different methods to frame out your walls. I've been building teardrops for almost 25 years, now, and my personal preference is to use a 1/2" or 5/8" plywood core cookie-cuttered like the wing-rib of an airplane, then sheathed on the inside with either 1/8" or 1/4" paneling, fill the voids in the core with sheet foam insulation, and then sheathed on the outside with 1/4" luan before skinning with aluminum. You might find a method that you like better or feel more comfortable using.
Good Luck on your upcoming project! In the meantime ...
CHEERS!
Grant
Celebrating
Retirement after over
32 Years of Building, Promoting, Supporting, Supplying, Living the Lifestyle, and Loving
Teardrop Trailers!"Life Moves a Little Slower When You're On Teardrop Time"The nature of Life, itself, is change ...
"Those who matter, don't mind, and those who mind, don't matter." 