Scan the member classifieds on this site and you can usually find some deals; I recently purchased an M Performance Rear Diffuser for a very good price.
People part out cars all the time and that's usually a good opportunity for buyers.I can't speak for all of the forums but the M5 forum has an active members classifieds section.
Good luck.
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CURRENT: 22' INDIVIDUAL M5| CHALK | TARTUFO | CCB | MPE | H&R | IND
GONE: 19' M5C| SINGAPORE GREY| ARAGON|VELOS VLS-06|H&R|iPE|IND|XPEL|CF|RADENSO
GONE: 15' F10 M5|BSM|BLACK|VELOS VSS-S2|KW|AKRA|IND|CF |ESCORT 360|VELOS PIGGY
GONE: 13' F10 M5|SG|BLACK|AKRA|KW|IND
LONG GONE: 09' E60 M5|02' E39 M5
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