I can't help you locat a supplier but FWIW, 10 ft sheets of ply are rarely
available. Plywood mills produce only 4 X 8 ft sheets and all of their
machinery is basically limited to that size. (I live in the area of several
plywood mills here in the Pac. NW.)

For my Ingrid rudder, I needed 12 ft sheets of 3/4" and finally found a
wholesaler that had scarffed sheets, done by a third party. The scarf was
well done but only about 8" long. Since my rudder was to be made up of a
lamination of 4 layers of ply, this was not a problem as long as I staggered
the these scarfs.

I doubt that you will find 1/4" ply already scarfed and suggest you just do
it yourself or find a way to build your project with butt joints in a non
critical location.

My opinion, FWIW.

Steve
s/v Good Intentions