How to purchase a Microsoft windows 7 product key only?

I want to buy the Windows product keys only. I do not need the install disk, as I have one already? How can I get just the cd key from MS?

It very easy just go here:
Windows 8 - Microsoft Windows

When not just go to official website:www.microsoft.com?

Be very careful with just buying the key, many blogs and websites have reported problems with the online upgrades when they have purchased just the serial key. But it is cheaper!

You can buy Windows 7/8 in a digital download and the product key from Microsoft's oonline Store. Link here: Microsoft Store - Free shipping. Free returns. On everything, every day