As sparksy said,
If you can track down a genuine windows 7 or 8 disk, you can download and install windows 10 for free, all you need is the licence number, even an old dead system with the windows sticker on it will do (must be 7 or later)
Technically you aren't supposed to use an OEM license as is restricted to the original computer, but I have seen plenty of them used without any issues, Microsoft doesn't seem to care as long as the computer is not being used commercially.
At one stage there was upgrade disk from win 7 to win 8 that were sold at Hardly Normal for $50, they worked OK as well