If you have previously "purchased" or downloaded Yosemite with your Apple ID, it will appear in your tab in the App Store. From there, you can re-download the installer directly. Creating a Bootable USB Drive
If you need a complete DMG to create a bootable USB for a clean install on a Mac without an existing OS, you may need to use third-party sources. Since Apple has removed this version from the Mac App Store, the following resources are often used by enthusiasts and can be trusted when verified. mac os x yosemite 10.10 5 download dmg
Follow the on-screen prompts. This will install the OS on your current computer; instead, it extracts the file named Install OS X Yosemite.app into your Applications folder. How to Create a Bootable OS X Yosemite USB Installer If you have previously "purchased" or downloaded Yosemite