You can buy 16' plywood if you live in the right locations in either 4' or 5' wide.
But....it's just factory scarfed 8' pieces. I've bought lots of it for boatbuilding. I have a jig that makes scarj joints for plywood (and dimenional stock) and they come out great...but you can see the joints even if they are 100% smooth.
You can buy 5'x10' and 4'x10' MDO plywood just about everywhere in the country. However it has no grain on the face, instead it has resin impregnated paper that holds paint/fiberglass/epoxy extremely well but wouldn't be worth squat for a grain face.
My first teardrop I built with MDO because I wanted it 9'6" long. Just used one piece (no seam) and only had to paint it when done and I glassed the edge connection to the roof with 6" tape.