I looked REALLY hard, and you won't be able to find the legit one anywhere. Microsoft for some strange reason no longer makes any of this stuff. Even the controller bundle is apparently old stock that will eventually run dry, so if you're going to buy, buy now.
Even looking on eBay, I only found one legit receiver being sold without the controller, but it was in a bundle lot with two sealed PC games and was sealed as well. Kinda pricey.
That is very strange and disappointing since it seems like the 360 controller was/is starting to become a standard option for PC games which I think is extremely excellent. Maybe the demand wasn't high enough at first but I would think it would be increasing now.