You can purchase Open Value subscriptions for Windows Server and SQL Server with Software Assurance directly from Microsoft or from a Microsoft reseller partner.
To purchase directly from Microsoft, you can visit the Microsoft Store for Business or contact the Microsoft Sales team. To find a Microsoft reseller partner, you can use the Microsoft Partner Center.
When purchasing an Open Value subscription, you will need to select the appropriate SKU for your needs. The SKU will depend on factors such as the number of devices you want to license, the level of software assurance coverage you require, and the specific edition of the software you need.
It's important to note that the process of purchasing licenses can be complex and may require consultation with a licensing specialist to ensure you get the right licenses for your business needs.