Not sure if Amazon is just working through the remainder of their stock though if it is discontinued (they're listed as a seller on the right since some third part one is like a dime cheaper). I never was able to find the Microsoft dongle alone without the controller though so perhaps they did discontinue it.
Not a bad price for both the dongle and a controller though and always nice to have another one for when friends stop by for some Sonic Racing.
https://encrypted.google.com/shopping/product/99842391963509...
Download the discontinued device's MS software.
x64
http://download.microsoft.com/download/0/0/f/00f2a67b-e10b-4...
x32
http://download.microsoft.com/download/0/0/f/00f2a67b-e10b-4...
Extract .exe with 7-zip
Go through the Device Manager>>>Update Driver on unknown device>>>Browse...>>>Let Me Pick>>>Have Disk>>>Drill down to Xbox360_64Eng\xbox360\setup64\files\driver\win7\x64
You may need to add the chinese device's hardware ID to the inf file...it's a buggy install but works flawlessly once Windows accepts the driver, just don't remember what the final trick was. JFGI.
Edit: clarified 'buggy install' & remove color commentary