If you're desperate, offcuts from the granite worktop fitters *can be* surprisingly flat... And free, my kind of price!
(I have a proper cast-iron one though - check EvilBay)
Dave H. (the other one)
I have a big piece of old granite worktop and as you say it is surprisingly flat, certainly flat enough for anything I do.
%&*%$ heavy though, I still have to make a stand for it!