Assume relatively low volume of 7-series is part of the issue.
Also, that the car is assembled in Germany.
I have a similar problem. One of my rear headrests is damaged (from child car seat).
I inquired about purchasing a replacement and was told there are NONE available anywhere, and BMW won't let my dealer order one.
I have an excellent rapport with my dealer and do not believe they would misinform me, but just to be sure I spoke with two other dealer parts departments also and got the same answer.
None available and can not be ordered.