How can I find suppliers of instruments that measure the volume (cubic
meters) of sand and gravel inventories? The volume of the piles are
between 200 m3 to 60000 m3.

I work for a minerals processing company that stockpiles various
grades of bauxite, clays, etc. We have tried various methods over the
years for determining consumption, with only limited success. We get
the most consistant estimates by using simple surveying tools, ie, a
transit and tape measure. Knowing the angle of repose of your
materials, height, moisture content, PSD will refine your estimate.  I
would love to know a better way myself.