I got it from microsoft's website. You can find it here: http://www.microsoft.com/windows/windows-7/

I will personally add somewhat the same caveat that they have, even though my experiences with 7 have been good. Don't jump on this as your day to day operating system until you know it works for you. I find it works wonderfully for me, but it is in the end still unsupported pre-release software, and will expire eventually.