In article ,
Phisherman wrote:

I need a 12-foot by 8.5" piece of 3/8" ply. The box stores only carry
8-foot ply stock. Already I have a 4x8' sheet of 3/8" ply. Is it
possible to make a 12-foot piece? I have most common tools.

http://www.eagleplywood.com/ says they make up to 5' x 12' panels (and
they're not the only one). Any good lumber yard (i.e. not Home Depot)
should be able to locate and order some for you. You might pay a premium
for it.