If your looking to buy a key just do a search on line. They're like $15.
Ah, 15 bucks in the good old US of A, heh. There are some online places here in Oz charging upwards of $150 - $200 for Win 7, Vista and Win 8.1. That's for a link to download the Windows edition from Microsoft itself, and the product key sent via email.
Funnily enough, I recently purchased a Win 10 Pro product key online for $99.00... a good $100 or more less than these older Windows versions. It seems so stupid that sellers are charging such high prices for older versions when nobody really wants them anymore and the trend is towards Win 10. I mean, if they really want them to sell, make the price attractive enough to the budget conscious and bargain hunter alike.