An old trick sometimes used was to look on Ebay at laptops for sale. very often when they take a photograph of the serial underneath, they inadvertently show the key. Chances are they wont but who knows, might be worth a perusal. (Smiles)
My thoughts
You used to be able to sign up to the Windows insider program (still can I think) and you get Windows 10 pro free legally plus you get to try out all the new features before the retail updates, and all the bugs too.
My spare machine has been using it since before windows 10 pro hit the shops and still gets the latest versions.