Activating Windows 7 help!

I am attempting to build a retro videogame machine using Windows 7 Ultimate x64. I got a copy of Windows 7 from a friend but the key he gave me is apparently blacklisted like he gave it out to a bunch of friends before me. Or he just used that key on every computer in his house when he used it. Anyways, is there a website I can buy a legit windows 7 key? Or is there a trick to getting windows 7 to activate since both the online servers and phone activation do not seem to be accessible. Thanks in advance!