I've looked into this some while ago although the situation may have changed.
The seat foam used to be made by Dunlopillo to a specific shape of the manufacturer and the moulds are no longer in existence.
Coach trimmers get round the problem by cutting foam by hand.
Suppliers are fairly plentiful like
www.efoam.co.uk who supply small quantities.
The trick is getting the right density as there is no standard measurement of résistance and phrases like soft, medium and hard do not give you any real idea of what you're buying until you can try it. I did it by asking for samples and layering differing density foam until I got something with slightly more resistance than the old foam. The difficulty was cutting it to shape which I did with really sharp scissors and a scalpel after trying heated wire and other foam cutters.
It took three attempts to get it right and in the end I wished I'd used a professional!
Later on I found that 'amalgam foam' would have been better (ie lots of small chunks of different density foam as a single sheet)
Maybe someone at BOC has made a template which would have saved me much frustration.
Good luck and let us know how you get on.