Here's how to:
but it's for 2002
Should be easier for e21
e24 seats are scarce et costly, much of the time the owner don't want to part their car or the trouble to do so.
But often you can buy the whole car and still be on the money.
I have seen some 2002 with e30 M3 seats ( frt +rr and even the cardboard )
e30 M3 int. are more than difficult to get but that's an indication that
e30 seats should work. There's plenty of them. the sport leather seats
were standard on "is" e30. (even in red )
The e30 rear bench is less sexier then the e24, but you can have one with
a folding armress wich is kind of cute. Be aware that e30 seat( frt+rr) have vertical stitches. e 24 and M3 have Horizontal stitches.
Sorry for the lengh.