Because Microsoft no longer sells or supports Windows 3.1, and it is , you won’t find a legitimate ISO from Microsoft’s official website. However, you may find preservation copies on reputable archival sites such as the Internet Archive (archive.org) .
It’s important to clarify that was not distributed on a bootable CD‑ROM in the modern ISO sense. It originally came on floppy disks (3.5” or 5.25”). Later “ISO” files you find online are typically custom‑made, unofficial disk images that bundle the installation files, often with DOS boot sectors.