I bought an imported 9x12 granite plate a couple of years ago from Grizzly. It came with an inspection certificate that states it was checked with an Auto Collimator that can optically detect errors of 0.000005" and found to be accurate to +/-0.0001". The certificate was issued by mhc Trading Co., Inc. I have been using it and have had no problems or complaints. Grizzly has pretty good freight rates but they are more than the price. They have three locations across the US and if you can pick it up it would be a super deal. I believe other suppliers also have similar imported plates, quite likely from the same Asian source.
I have read about several substitutes for granite surface plates. One suggestion was a polished slab from a local monument maker. I doubt they can furnish a calibration certificate but the product seems to be quite flat. Glass has also been mentioned: I forget what kind or the source.
Make it fit.
You can't win and there is a penalty for trying!