I wrote a guide on repairing w123 seats (I think they are basically identical to yours). This is good if your seats have any broken springs, also I added foam inserts from a 124 or 126 - not sure which exactly. This wont bring the seats up to the firmness of modern car seats, but makes them much much more comfortable than broken down ones. I am sure other seats will fit, but will almost certainly require modifications to the seats and or floor pan.
Total cost was well under 100$ for both front seats, though my passenger seat was in fairly good shape.
http://www.peachparts.com/shopforum/detailing-interior/373355-seat-repair-alternative-technique-w123-similar-seats.html