Any legal/Microsoft way to make Windows 7 installation legal by purchasing a key for it [duplicate]

I searched a lot and there are resources which point to e-bay and others but all that is third party.

I would like to buy JUST THE KEY for Windows 7 NOT the media, so I can make the already working copy of Windows 7 legal/activated.

Reason for this is to save some money. I have the media just give me the key, and only charge me for the key.

Buy the Download version:

http://www.microsoft.com/windows/buy/default.aspx

You should receive a key without having to actually download (although keeping a copy of the ISO will always be handy!)

You can buy a upgrade key at any Best Buy, but for a new copy of the same version you need to get the media per Microsoft:

http://windows.microsoft.com/en-US/windows7/Get-a-new-Windows-7-product-key