Taking into consideration that a E90 is a entry level BMW and parts are readily available, and most of all, cheap, maybe consider to buy new disks?
I called around a few months ago and at the time OEM disks were just over R3000 for a set of 4 (BMW JHB city had a special).
It's a critical function of your car after all, and it doesn't require frequent maintenance.
Edit: I recall a fellow member selling a set of used rear disks a few weeks ago. Maybe also look at the for sale section.
If I recall they were off a 330d.
Sent on the move via Tapatalk