If you can't find iron, I'd use some hot rolled alloy (IE 4140) steel - easier to find.
First have it annealed and normalized, preferably while hanging vertically in a neutral salt bath - less chance of warpage. Furnace cool or slow cool as much as possible
Leave in heat for 1 hour per inch of matl thickness. Normallized, it should be Rc 26 or so which is close to cast iron.
Then find someone who can blanchard grind it and who's prepared to shim it to get it flat. You can't just suck it down as it will be flat when you grind it but will spring back when you release it.