Where can I find a replacement bumper cover?

I have a limited edition lunar white XBone controller and the left bumper is broken (the cover, not the internal button itself). I love the controller, and would love to replace the button cover, but with the new all-bumper-and-sync-button-covers-are-one-piece crap the spare parts I have for that are useless. Does anyone know where I can find a replacement for that specific piece?