Best Sources to Purchase a Legitimate Windows 10 Product Key

I'm planning to install Windows 10 on my new 14th gen build and need a valid Windows 10 license. I'm hesitant to buy from cheap resellers because their keys often fail after a few months, especially following a reinstall. I've noticed that Microsoft no longer sells Windows 10 keys on their website, only Windows 11 licenses are available. Does anyone know if Windows 10 and 11 keys are interchangeable, or will I need to find an older, genuine Windows 10 product key?