In MMD 21.11.30, Wallace Venable presents a discussion of types
of woods useful for sounding board construction, along with a few
non-specific sources.

A possibly larger, if still non-specific source could be salvaging
and recycling the sounding boards from the scores of derelict pianos
that are transferred to local landfills every day (with the possible
exception of Sundays). For instruments at least 50-60 years old, the
material is likely to be Sitka spruce, the go-to material for the
purpose.

Extraction can be a bit of a chore (I've done it for a guitar maker
friend) but the results were more than satisfactory. If necessary,
a well-adjusted planer, using minimul-depth cuts will reduce it to
the thickness needed.

John Grant
Maryland, USA