It used to be the case that the only option to refurbish the XTZ calipers was to buy BMW parts from Motorworks (it's all in this thread), but luckily in the last year Powerhouse on ebay are doing complete kits.

Front (one caliper): item235063e1b5:g:2W8AAOSwl8NVaEkV

Rear: item2351d8de90:g:NE8AAOSw3xJVaE24

If you only want individual parts, say just the rubbers, then Motorworks is still the only way to do it. However, it's only a little more money to get the full kit anyway from ebay.

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