Yes pay cash for a key.... no other way to 'get it'
Either buy a windows 7,8 or 8.1 key install that then upgrade to Windows 10 for free or just go and buy windows 10 out right.
Buying the windows 7,8 or 8.1 keep will be the cheaper option i would imagine.
This is the only way now that I can see to solve your problem - should have kept your windows 7 license in case of trouble - lesson learned the hard way.