Circle BMW is one place. You can also get them from Crevier BMW's Online parts.

http://www.circlebmw.com/parts/sirius/sirius.htm

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SOLD
2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input