Where do I get a flattening plate?
Note: Links in this article are for UK
It looks like the thing to search for is a granite surface plate. The other alternative is float glass lapping plate, which can be cheaper, but you have to make sure it is thick enough not to flex. One example is this one. Make sure you get one of either that is at least 12” x 9”
We also recommend counter savers/protectors made of granite that can sometimes be had from Aldi. You do want to check them for flat with a straight edge, but they are very affordable. Paul has also purchased granite tiles from B&Q before that he checks with a square that he has picked up in store.