the seat pad part no. comes up as A12339102650 for a US 123 300D - this part no. is to be used with the mbtex material. I cannot seem to find a price for it though. The later cars had the orange foam on the seat bottom while the earlier cars had the rubberised horsehair pads on the bottoms.
To repair the spring box, you would need to use some scrapped diesel injection lines and drill them to slip them over the broken springs so they become one piece again. MB also have an upgraded pad (green in colour) and also corner supports and centre tubes to assist the old seats.
How come the MB dealerships couldnt get it, next time give them the full VIN and ask that you want to see the screen. Its in plain view if you look at the front seat diagram, on the 123.130 model the callout no. from the pictogram was 17.