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I think if you GOOGLE it you may find several places that sell them, and if your looking at this post look to the left the AD may lead you in the start direction for your needs.
Actually most people say that the OEM is sufficient enough. Blew my mind too. Just look at the "modular V10" forum. Search in there and there's a thread or two that go into detail.
Plus, I really havn't found too many aftermarket ones worth looking into.