I wonder if you contacted your local/closest BMW dealership and had the part dept look up the BMW part nbr for the rear mud flaps...they may be able to delve deeper into the parts data base to see if there's a part nbr for the hardware that comes with the mud flaps when you purchase them.
For instance, the part nbrs for the hardware may be included in the ASAP (after sales assistance portal) information that they can print out when you purchase the mud flaps.
Those same rivets & push pins may also be used somewhere else on the E53 X5...or even on some other BMW model. But if anyone was able to find out if the hardware was available...a BMW dealerships parts dept should be able to do it.
Good luck.
