Earlier this week, the Microsoft Store was updated with a new ‘Xbox Repair & Replacement Parts’ page, offering replacement parts for the current series of Xbox Wireless and Elite Series 2 controllers. Replacement parts include top cases, PCBs and motors, and replacement buttons at of this posting. It wouldn’t be too surprising to see more added to this page over time, especially since the subtitle of the site uses the all-encompassing ‘Xbox accessories’ term.
Check out the Microsoft Store page here. At the moment this page appears to be for the US only.