I'd normally default to Amazon, but this looks slightly dubious: http://www.amazon.com/Wii-MotionPlus-Nintendo/dp/B001TOQ8NO/ref=sr_1_1?ie=UTF8&qid=1319934852&sr=8-1
At least, there's one review that says it was fake when they got it. Would it be safer to buy a WiimotePlus?
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Now playing: Saints Row 2, Final Fantasy IV (PSP), Ratchet & Clank