Options for getting a really flat build surface
It all depends on what you mean by "flat".

Is the problem that the build plate isn't flat (perfectly planar), or is the problem that the distance to the build plate varies based on X and Y coordinates?  They are very different problems.

"Bed Leveling" is the process of allowing the firmware to know the Z position of the build plate for every (X,Y) location.  Some printers support measuring a mesh of points and interpolating the mesh.  This compensates quite well both for warped build plate, and for bad positioning mechanisms which add some position dependent Z-axis offset.

If you really need a flat, planar, build surface, perhaps because you are printing mirrors or precision parts, glass should work well.  It is still, and will not brook any inelastic bending.  It will shatter before it takes on a curve.  If possible, you could consider a Pyrex® bed, since it has a lower coefficient of thermal expansion and won't warp as much with a temperature gradient through the glass. 

I tried to find a spec on how flat your glass might be.  I'm assuming it is "float glass", which is made by floating molten glass on a pool of molten tin.  I didn't find a spec, but I found this answer to a similar question:


  I doubt you will find such a spec- float glass isn't manufactured for
  that purpose and it isn't really in the manufacturer's interests to
  maintain such a spec. that said, the stuff is surprisingly flat, just
  as a side effect of how it's made. you don't say what your needs are
  other than near optical tolerances . might just be good enough for
  you. consider though that granite surface plates of certified flatness
  can be had fairly inexpensively these days.


So, it is clearly flat.  I can't tell you how flat.  And the flatness depends on not applying stress to the glass that causes it to bed.  Thicker is stiffer (probably at the third power of the thickness ratio).