Is it a diff for a 3L or Evo you want?
You'll only be able to get a 3.91 at the most for an Evo using an E34 M5 cwp. 4.1, 4.7 or 5.1 etc are only from BMWMotorsport but you'd buy another M3 for the cost of the cwp


If its for a 323-3L M3 subframe, Buy an E30 325 3.91 LSD and fit the internals into an E36 open diff and away you go. Worth putting new plates and uprating the lock putting it into an E36 though.