Processes in Microsoft 365 Insider for installing preview builds, activating subscriptions, and keeping apps up to date with the latest Insider features.
>>
from where I would buy key for all three versions (2010,2013,2016)
<<
None of the those are still sold by Microsoft so you would have to find a 3rd party reseller such as Amazon. Word of advice, be wary of vendors that offer products at prices that "appear to good to be true" since those usually are. Unfortunately, there are vendors out there that sell Office with keys that are from MSDN or Volume License Agreements that are not legally allowed to be sold and may suddenly become inactive at some point in the future if too many people have been sent the same key.