You might want to go to an upholstery supply house....
They have foam in three densities typically....
It can be cut with a knife... or an electric knife can do a sculpting job..
Even if this was not pretty enough for long term it might be more comfortable while you are looking for the others....
But to me it seems illogical to try to find seats which are pretty much as old as yours and expect them to be in much better shape...
That horsehair can be replaced with modern materials...
That horsehair was used for buggy seats in the 1700's... and I am not being funny..