For the smaller pipes you might consider old pianos.  These soundboards
are made of spruce about 1/4 to 3/8 inches thick.  If that thickness
can be used in organ pipes there is an abundant supply.  Call your
friendly local piano technician.  I am sure I am not the only one with
20 or so "hulks" gathering dust.  I have never figured out a use for
old spruce soundboards from discarded pianos.

Richard Moody, Piano Tuner Technician
South Central South Dakota