"This item is temporarily unavailable" When Installing Yosemite
Let me preface this: it's possible that this solution doesn't work entirely, and that you'll need a version of the installer that has a full copy of OS X (instead of downloading it from the internet on the fly.)

With that said, here's the issue. Your Apple ID (and therefore purchases) are not associated with the computer you're trying to install to since it's a clean install. Therefore, it doesn't know you've "purchased" Yosemite previously, and since it's no longer on the app store, isn't giving you access. What you need to do is reassociate your Apple ID with that computer. The recommended solution is to log into icloud.com.

Alternatively (or if that doesn't work), you can download it from another computer with a working version of OS X and then execute the installer from there (or create a bootable installer from there.)

Small side question: why not install El Cap?