For replacement upholstry you can use
World Upholstry or
GAHH.
Other seat parts such as springs and padding can be ordered from Fastlane.
When pulling seats from the yards, be aware that in certain years Mercedes used a padding that disintegrates into red dust. I'm certain of 1980 because my 240D has them. Maybe 81 and 82 as well. Just beat on the seat a few times. If red junk comes out the bottom, then the padding won't last long even if it appears to be perfect.