One option is to not worry about seat covers. When they look shabby the originals can be removed and washed in the washing machine. While they're in the wash you can repair or replace any seat foam that's falling apart. Removal instructions can be found here
http://cleanflametrap.com/tony/seat_bottom.htm
http://cleanflametrap.com/tony/seat_back.htm
It's not as daunting as it looks in the instructions -- I found it quite a satisfying job to do.
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Present: 1990 240GL saloon, 1992 240 estate
Past: 1988 240GL; 1971 144DL; 1972 145DL