Flat as a (very precise) pancake...

Is it possible to get Precision Ground Flat Stock that is more
precise than the usual off-the-shelf stuff?
I'm talking about parallelism somewhere in the neighborhood of +/-
0.0005" per foot in width and thickness.
I've noticed that the Starrett products that I have laying around are
noticeably warped. I'd like to know where one can get flat stock that
is actually flat. Is this a specialty item that needs to be made to
order at high cost?

Tom Murray