where exactly do I get a windows 10 product key?

[Matt1403]

hi,

I've recently built my first PC and to save a bit of money i went with (free) windows 10 for the OS. I downloaded the iso file from Microsoft and put it on a USB drive then installed it using a generic product key. it all worked fine for the first couple of days then I started getting thoughs annoying prompts to activate windows. i looked around on the internet and found couple CD keys none of which worked id even tried guessing a couple times with no luck.
so eventually i gave in and thought id get one legit but after looking around for awhile i couldn't find any sites that sell windows 10 product keys

so my question is where can i purchase a legit working product key for windows 10?

although the prompts have seemed to stop (for now) it still says not activated :/

thanks,

 

[Someone Somewhere]

Windows 10 is a free upgrade, only if you already have a copy of 7/8.1 installed.

Direct from MS: http://www.microsoftstore.com/store/msusa/en_US/pdp/Windows-10-Home/productID.319937100?VID=320028200&ICID=Win10Home_DualCTA

 

[USAFRet]

so my question is where can i purchase a legit working product key for windows 10?

Microsoft/Amazon/Newegg....