12 bucks for a fake one:
http://dx.com/p/pc-wireless-gaming-receiver-for-xbox-360-controller-black-129230?item=7
http://dx.com/p/pc-wireless-gaming-receiver-for-xbox-360-controller-white-grey-129241?item=18
Free shipping from China, which means it'll take a month to arrive.
I have both, a Fake and a Original one. There's virtually no difference between them. You MAY need to input some drivers for the fake one, but they come in a CD and I didn't (maybe because I already had the original drivers installed before, I'm not sure). They report as being the same to Windows, so there are no game incompatibilities.
You can't get original ones by themselves new anymore. They're only bundled into "Games for Windows" controller bundles (which is just a 360 controller and a wireless receiver).
You might get an original used one on eBay or Amazon, but they have the shitty habit of blowing up a shitty fuse inside them, rendering them unusable. You can easily fix by gaping the connection between the both solder points. I just did it yesterday to one of my old ones.
Since they're so hard to find (and I doubt the fake ones even have that fuse), I'd tell you to buy the fake ones from China. It'll take a while, but you can buy two at a time and never have to bother with it again (because if it's breaks, just swap to your other one, and order a single one as a backup again).