Need a Marble or Granite Slab (concord / pleasant hill / martinez)

Looking to purchase a piece of Marble or Granite slab that is smooth/polished (not necessarily high polish) about 18" by 24" .
Any thickness is okay but the thicker the better.
Shape does not necessarily need to be exactly the stated dimensions, but something close, and doesn't even have to be rectangular.
If you know where to acquire something like this please contact me and let me know how much you want for it