Got loads off ebay, I think £4.99 is the most ive ever paid for one, in fact im using one right now, never had a problem re-activating it after a full format of my drives etc, brought my Dad's off there too, still going strong, and many other machines ive built or upgraded too.
Id suggest you buy a Windows 10 Pro key though and download the ISO for the Pro edition then upgrade to that instead, you'll have more control over your machine.