I know I can get it from our local plywood specialty shop. But it was tough finding them. I ended up calling 3 lumber yards before one of them told me about Pontiac Plywood.
Pontiac Plywood has it shipped in from a distrubutor, as I imagine someone in your area might. You might want to call around and ask who sells speciality plywood or oversized plywood.
As for building a weekender for the Oct gathering it's do-able. Here's what I see...
You already have the trailer with running lights and brakes...
You build a body out of 3/4" plywood and gorilla glue, re-inforced with wood strips on the ceiling where necessary, and aluminum angle brackets if you feel it's necessary. I ended up with 1 wood strip on the mockup.
Paint it with CPES and Uniflex 255.
Install a battery powered closet light and you're ready to go.
Am I close? Do you have a trailer already?
Mike...
P.S. You can always come back and make it lighter!!!
P.P.S. If you built it out of birch plywood, you're looking at $344 for the body. That's not too bad... I wonder if one could use 5/8" BC fir plywood?
P.P.P.S. If you can't find 4x10, then use 4x8 and use a batten.

The quality is remembered long after the price is forgotten, so build your teardrop with the best materials...