I own an E39 and E53...
I go to junk yard and pulled a few FSR from other BMWs such as E39, E46 etc.
I keep them as spare.
The junk yard charges $10/each.

Or you can buy a few used FSRs on ebay (make sure you buy used BMW parts, not aftermarket stuff!). A used BMW part is 10x better than aftermarket junk.

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT