Apple made Mavericks completely free . Because it was free, Apple never sold it on a USB stick or DVD. Consequently, an official "ISO" file never existed directly from Apple. Apple uses the .dmg (Disk Image) and .app (Application bundle) formats.

You usually don't need a bootable USB for an old Mac (you can use the .app file). You want an for one of two reasons:

If you prefer a more automated solution, the script shared on the VirtualBox forums can handle the conversion for you. It supports multiple versions including Mavericks (10.9.x) and doesn't require administrator access for this specific version. Method 3: Creating a Bootable USB on Windows

Let’s break down why the ISO format is rare for Mavericks, and—if you genuinely need one for a virtual machine like VMware or VirtualBox—how to create a legitimate one yourself.

hdiutil convert Mavericks.tmp.sparseimage -format UDTO -o Mavericks.iso