Interesting discussion. Highly unlikely that they could have made you a one off at a price you'd want to pay, but it would have been better all round if the person on the phone had explained that.
As Inspector said, for forging, the expense would be in the dies, and nobody makes an ordinary chisel wider than 2".
I've looked back at 19th and 20th century catalogues and can confirm that that's always been so, except for timber framing or shipbuilding, but they are really a different tool.
However, briefly, there used to be an exception. How do you like these from the 1928 Marples Catalogue?
No puny 2½" size here - how hard would you need to push to use a chisel 9" wide?
The answer (somewhat disappointingly) is that (as far as I can deduce) these were for cutting leather, not wood. Specifically, they were for cutting accurate bevels in thick leather which was stuck on rollers, used in cotton spinning mills. There used to be a specialist trade in maintaining the thousands of rollers. You can read a little bit more about it here:
http://www.taths.org.uk/queries/170-the ... r-s-chisel