I usually get them from eBay for $10 - $30
Just make sure the seller is legit and has feedback.
You can get a legit key with a DVD for around $30
I personally wouldn't use the DVD since it's probably out of date.
You can always create your own install media here
https://www.microsoft.com/en-us/soft...load/windows10
Or just use the DVD and update, no big deal either way.