Where to get an official Big Sur 11.2 ISO image that can be used for VM?

In addition, Apple does not distribute "ISO Images". Only full installers are available and downloaded from the Mac App Store, which will only give you the current version.

However, within the installer package there is a utility to create a USB installation media.

How to create a bootable installer for macOS - Apple Support

As others have mentioned.

Or if you kept a Full Installer of 11.2 and / or 11.3 on an external drive Before running the Installer.

Have a Bootable Clone of the System with Big Sur 11.2 and / or 11.3 hanging around ??

Here are the available full installers. There is no such thing as a macOS ISO.

I think 11.3.2 was available only a few weeks ago, but this is all I get, now:

$ softwareupdate --list-full-installers
Finding available software
Software Update found the following full installers:
* Title: macOS Big Sur, Version: 11.5.2, Size: 12440916552K, Build: 20G95
* Title: macOS Big Sur, Version: 11.5.1, Size: 12440158909K, Build: 20G80
* Title: macOS Big Sur, Version: 11.5, Size: 12443337213K, Build: 20G71
* Title: macOS Big Sur, Version: 11.4, Size: 12395342506K, Build: 20F71

There are no Big Sur ISOs and 11.2 & 11.3 are no longer available. The current version is 11.5.2.

Some fee-based, Virtual Machine products (e.g. Parallels) can read the macOS full installer's .dmg image and perform the guest installation. What others have said about no ISO format.

Just to add, depending on the app you are using for the VM, you can mount the USB installer in the VM and go from there. I know Parallels and VMware Fusion can both use the full installer.