Where are you supposed to purchase Windows Server 2019 RDS CALs?

Solution 1:

RDS CALS are generally "backward compatible" in that a Server 2022 RDS CAL is valid for a Server 2019 RDS host (and probably also for Server 2016, Server 2012 R2, etc.). While I can't find Microsoft's specific article stating as much, it's a safe bet that you can purchase Server 2022 CALS, install them on a Server 2022 RDS License Server, and use them for your Server 2019 RDS hosts.

As for where to buy them; use a known reputable reseller like Ingram Micro, CDW, etc. or purchase them directly from Microsoft.

https://www.microsoft.com/en-us/d/windows-remote-desktop-server-cal-2022/dg7gmgf0d7hx