Visit the IBM Entitled Systems Support website and sign in with your IBMid.

Your IBM Power Systems hardware must have a current and valid maintenance contract linked to your IBMid.

64-bit Common Hardware Reference Platform (CHRP) machines running POWER8, POWER9, Power10, or Power11 processors in POWER8 (or later) compatibility mode.

Search for AIX V7.3 or select it from the list of operating systems. You may need to select your specific machine type and serial number if prompted.

Once the base OS is installed via the ISO, you can maintain your system using these secondary sources: AIX 7.3 Release Notes - IBM