Q: Where can I find the Yosemite Installer?

Make a bootable clone of your existing system and/or use the Disk Utility to repartition the internal drive. If it matters, a fresh installation of Yosemite can be performed by restarting the computer with the Option, Command, and R keys held down and choosing to do so, but it isn’t possible to make a copy of the actual installer used.

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I've seen where I can purchase a backup disk on Amazon, but I would REALLY hate to pay for something that's meant to be free.

It would also not really help as the licensing agreement states that any OS or app downloaded from the app store is NOT transferable, so you'd be paying for what would essentially be a pirated copy.

You can follow Niel's suggestions - you'll wind up with a bootable clone of your system or use recovery and download the OS installer from which you can make your own bootable copy.