I believe you received the correct information that the i8 coupe does not have rear floor mats. None in my 2015 and none shown in the REALOEM parts catalog.
There is a USA company that makes rubber floor mats for the i8. Both front and rear are available. Rear only are $59.95 USD. Front and rear are $169.60 all are in black rubber.
https://www.weathertech.com/bmw/2015...er-digitalfit/