i always thought they were slate? At least every table I've ever put together, including a 1908 Brunswick my father and I restored were all slate. It used to be significantly thicker than they are made now, however.... I had to go find my stones after carrying down the pieces for the 1908! But i've also worked on tables from the 50's, 70's, and 90's, they were all slate as well.
Of course, you are correct, I was thinking slate but wrote granite. They even call them "slates".
I found it interesting that they stopped just throwing out the chipped or otherwise defective slates about a month before it dawned on me to go looking for them for a build surface(which, as a point in fact, was about 1985). It's like stunt engines, by the time we can decide they are really good stunt engines, they are discontinued.
A good possibility today is someone's old granite or marble kitchen counters. I don't know about anywhere else, but redoing the kitchen every 10 years or so is a very common thing around here. They just go in and knock out the old countertops with a hammer and toss the remains. A kitchen remodel is about $100000 or so, typically, the cost of the materials is such that no one bothers to salvage it.
For most purposes, even a 2' by 4' section (out of a 32" x 8 foot countertop) is useful, so just find the biggest chunk, cut it off square to suit your OCD/perfectionist desires, good to go.
By the way, all joking aside, Dan's right - that is not a bad cost at all for a granite surface plate, and there are a lot of them available as precision machine shops go out of business. If had a place to put it in 1986, and could have gotten a similar deal (about half price of a new one in 1986 dollars), I would probably have gotten one. It's overkill, I would bet that a Grace B or lower is much cheaper than a Grade AA or A, even less without a recent inspection/calibration certificate. I am slightly tempted to go get one and store it for my post-retirement move, but I am going to be dead in 25 years or so, so why bother?
Of course, any of these graded measurement references is great overkill. I think I can get the "no load, thermally stable" alignment measurements to something like .015-.020", about the width of a fine pencil line using pretty conventional methods. Beyond that, no, and it doesn't matter because as soon as the sun hits it, and even more so, you take your nice straight-to-0.015" wing and then put 60-70 lbs of load on it, it's going to flex by 1/2-3/4" at the bare minimum. Selecting the wood grade and the structural design to improve symmetry from one side to the other is much more important than measuring it to a few thousandths, even if you could.
Brett