I suspect the problem they are having is that plate (its not sheet) of that thickness doesn't come in 2m x 2m, to get a 2m x 2m piece you will need to buy a 4m x 2m, which is twice as much as you want, and cut it down,
Its not that common either, the last 4x2 heavy plate I was involved in getting hold off took three weeks or more to arrive.