I see you are looking for the lens barrier that is part of the lens hood. I have searched but so far have not been able to come up with a part number for the "lens hood with lens barrier". I searched the manuals for the HF G20 (which I have) and the HF G30 and in both the part is listed as lens hood with lens barrier and no part number. Your best bet may be to contact Canon directly.
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http://shop.usa.canon.com/shop/en/catalog/contactus
