Do you know anyone who has purchased Yosemite from the App Store? The reason I ask is you can get the file from their Mac, even if they have updated to El Cap, just by going to the Mac App Store downloaded section. Then you will be able to install it. Did you update to El Capitan? Are you trying to revert back?
Most of my computer friends are still on Windows. The two that have macs are in the same position as I am.
This iMac and one of the MacBooks are still on Yosemite, my own Macbook is on El Capital to which I upgraded yesterday, but even though I downloaded it a second time, without installing it, I cannot find that downloaded file. I am beginning it think that Apple has changed its download policies!
Apple does not allow you to download previous OS X systems once a new one is released. If you need to reinstall Yosemite you can use System Restore, which is now internet-based meaning it will download system files online from the App Store after Apple verifies your computer.
My understanding is you need that installer file to create a boot USB or other external drive media. Just posted a similar question on the forum...Good luck!