The only place I can guarantee will sell you just a key and you can be sure it is not fake is Microsoft but it is very expensive. Your best option is to find a place in your country where you can buy a system builders license (it only includes one DVD and the key so be sure to get either the 32-bit or 64-bit DVD depending on which bit-size you need). A system builders license does not include Microsoft support but who really calls Microsoft about OS problems anyway, so it is worth the difference of almost US$200.