Since Yosemite is no longer featured on the App Store front page, you have two official channels:
Because Yosemite is considered an "obsolete" operating system as of 2017, it is no longer prominently featured on the Mac App Store. However, there are still official and community-trusted ways to get it: Official Apple Support Mac Os Yosemite Download Iso
Ensure the installer app is exactly named Install OS X Yosemite.app and located in /Applications . Spaces in the filename require backslashes: Install\ OS\ X\ Yosemite.app . Since Yosemite is no longer featured on the