Given the tight radius of curvature, that is probably "bending ply",
rather than regular birch/alder plywood. IE, the plies are laid up
more-or-less with parallel grain, rather than more or less 90 degrees.

If you need regular birch/alder/etc Aircraft Spruce is a good source,
though not overly inexpensive. Presumably of high quality, given the
application - and more plies per thickness than the stuff my lumberyard
gets - their 3mm is 5 ply, the 3mm from my lumberyard is 3 ply.

http://www.aircraftspruce.com/menus/wp/plywood.html

My local lumberyard does not stock birch ply, but has ordered stuff in
for me.

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