I went into this to some tune when i had mine - with Liquid leather - they reckon there are at least 4 Imola leathers so the only way is a swab of leather and they will match ( note M3 Imola is probably quite a different shade as the car is assembled in Europe whereas the Z4M in South Carolina)
the best location i found was at the back base of the seat if you feel down you come across a black clip which has the back of the seat under slight tension, beyond this clip there is a food couple of inches of spare leather.
note he needs quite a bit to get a match - he sent my first "sample back" saying it was too small to get a colour match from .