You can download Chrome OS from the official Chromium OS website. However, please note that these images are intended for developer use and can be unstable, so use them at your own risk.
Here are the steps you can follow to download Chrome OS ISO:
1. Visit the official Chromium OS website at Chromium OS Docs - Chromium OS Developer Guide
2. Select the branch you want to download. You can choose from Stable, Beta, Dev, and Canary.
3. Scroll down to the "Getting the OS Image" section, then select the appropriate image for your system. VirtualBox users should download the “VirtualBox image”.
4. Extract the downloaded file and then start VirtualBox.
5. Create a new virtual machine and choose the extracted Chrome OS file as the startup disk.
6. Customize the VM settings and start the machine.
That's it! You should now be able to run Chrome OS inside VirtualBox.