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Aero side skirts a la carte are available from any dealer. However I think they come only in primed not painted. I have the MINI accessory catalog in my hand and the Aero side skirt (primed) is part number:
51 71 0 021 932 (Left)
51 71 0 021 933 (Right)
Retail is $245 each. There is no listing for prepainted skirts only.
Good luck.
Classic will offer you 20% off of the retai price. That is the best deal I've seen.