I’ve actualy seen adverts for CDkeys no youtube i think, or it might have been one of the freeview channels, so I guess they must be semi legit, at least!
But as above, just buy a cheap one – you should be able to get one for about £5-£10.
I did actually have an issue with an office key from gamers outlet once – error that it was already registered and trying to get a refund was basicaly impossible so I gave up as it was only £3 if I recall.
But I’m running cheap windows keys on both my windows PCs, never had a problem. In theory I could wake up one day to a ‘this windows is unlicenced’ messsage, but in that case I’d just buy another cheap key and re-activate it.