It's not discontinued, it's just MS being asses. Instead of letting us spend $20 or less they shoehorn us into pay $50 for the receiver and controller, even if you already have some xbox 360 controllers on tap.
As for the knock-offs, I haven't heard any issues other than getting drivers to work / not wanting to use the provided drivers, for good reason;
http://www.selfsimilar.org/2011/07/counterfeit-xbox-360-wireless-receiver-drivers/
That said you can get an official one for under 20 at amazon -
http://www.amazon.com/dp/B000HZFCT2/?tag=neogaf0e-20
If you still want to go with the knock-off, this one has so-so reviews;
http://www.amazon.com/dp/B005F1ZUU4/?tag=neogaf0e-20
Just follow the following if you go this route;
1.Right click on Computer
2. Go to Properties
3. Click on Device Manager
4. Right click on the Unidentified Device
5. Go to Properties
6. Go to the Drive tab
7. Click on Update Driver...
8. Browse my computer for driver software
9. Let me pick from a list of device drivers on my computer
10. Choose "Microsoft Common Controller for Window Class
Xbox 360 Wireless Receiver for Windows Version: 2.1.0.1349 [8/13/2009]"
11. Accpet "Update Driver Warning"
12. Click Yes