New
#11
You can find Windows 7 on DVD on Amazon for around 100 bucks on up from various sellers.
https://www.amazon.com/Windows-Premi...6%3A1249173011
Here's one example. But if you look at the reviews, some claim it's a counterfeit, even though sold directly from Amazon.
I'm not sure how you could be 100% sure of getting a legitimate version any more since I don't think you can get it directly from Microsoft.
Newegg.com is selling Win 7 Pro for $140 and I'd be nearly positive that would be genuine.