Hi,
You can purchase Extended Security Updates through Volume Licensing Programs such as Enterprise Agreement (EA), Enterprise Agreement Subscription (EAS), Enrollment for Education Solutions (EES), or Server and Cloud Enrollment (SCE).
For more information you can refer to the documentations below:
How to use Windows Server 2008 and 2008 R2 extended security updates (ESU)
https://learn.microsoft.com/en-us/windows-server/get-started/extended-security-updates
Extended Security Updates frequently asked questions
https://www.microsoft.com/en-us/windows-server/extended-security-updates
Procedure to continue receiving security updates after extended support ends on January 14, 2020
https://support.microsoft.com/en-us/help/4522133/procedure-to-continue-receiving-security-updates
Best regards,
Leon