Hmmm...tough call. I can tell you that in a 500E the seat mounting points are moved outward, so as to allow for the widening of the transmission tunnel. Seats are further apart than any other 300, but not by a lot (maybe 2 inches?).
However, the 500E seats were also available in the 300E Sportline models, so I'm inclined to think that they'd fit...can't think they'd modify the mounting points for Sportline cars AND 500Es. But, sorry to say, you never know.
As to where to find them, start calling boneyards...they are really nice seats, I might add, and if I come across a set I'd look to pop them into the wagon or D.
"If everything seems under control, you're simply not going fast enough" --Mario Andretti
1995 E500 street car
1986 Porsche 944 Turbo S track car