It'll cost you about the same as a good pub lunch, comes with free express shipping and -- as Microsoft has clarified -- works on Windows 7, 8.1 and Windows 10.
It's available directly through Microsoft's local online store, where you can pick up the adapter for $20.96. That includes free express shipping and free returns too, bringing the total saving to just under $20.
The store description says the Xbox wireless adapter is for use on Windows 10, but Microsoft's official knowledge base confirms it's "compatible with devices running Windows 10, Windows 8.1 or Windows 7". Note that this isn't the same adapter as what Microsoft released for the Xbox 360 controller, which supports vanilla Windows 8 (if you're still running that for some reason).
Microsoft has also pushed out an update that was correcting BSOD errors related to one of the adapter's drivers, with the latest patches coming out earlier this month. (If you want to browse the previous link instead of going through Windows Update, you'll have to use Internet Explorer or navigate through the Microsoft Download Center.)
Plenty of console gamers and PC fans who wanted a really nice controller have been enjoying the Xbox Elite pad since Christmas, and this is the final piece of the puzzle to tie it all together. It's the cheapest you can get it for and you can't get much better than buying it directly from Microsoft. And sure, the technology in the Elite isn't as cool as what Valve is trying to do with the Steam controller, but that doesn't work out of the box anywhere near as well as the Elite pad.