Where Do I Get a Windows 10 Product Key in 2026?

Where to get a Windows 10 product key now that Microsoft has stopped selling it, how to check for an existing license, and Home vs Pro.
Buying guides for Microsoft software: which Windows edition, Office version or Windows Server license to choose, what a genuine key should cost and how to avoid gray-market listings.

Where to get a Windows 10 product key now that Microsoft has stopped selling it, how to check for an existing license, and Home vs Pro.