76 450 SL seat question

I recently had the driver seat in a 76 450 re-upholstered by a reputable shop. They completely overhauled the seat frame, springs, etc and replaced the old vinyl with leather. the problem is the seat is so firm, it is downright uncomfortable. When i asked them about adding more cushioning I was told that's the originally the way they made them. Someone else told me I could replace them with a 380 or 500 seat up till about the 1988 model and get a much more comfortable seat. Is this correct and if so,where do you find used parts on these in my area, Raleigh, NC?