Install Yosemite file
One way around downloading any version of OS X that's not supported on the physical hardware is to download it in a virtual machine, on the unsupported system, that's running a version of OS X that supports the downloading of the version of OS X being downloaded. I did this using VMware Fusion.  The virtual hardware presented in the virtual machine allowed for this as the App Store did see the physical hardware and the virtual hardware presented was adequate for the OS X version being downloaded.

I had to do this with downloading OS X Mountain Lion in a virtual machine on a physical machine (MacBook Pro 15-inch Core 2 Duo) which only supported up to Mac OS X 10.7.5.  I needed to do this to create a USB installer of OS X Mountain Lion to be used on a MacBook Pro Retina, 15-inch, Early 2013 to create an image of the SSD before the OS had been run directly from the SSD for the first time.  In other words to create a factory sealed image, so-to-speak.

Otherwise you'll need to do as Tetsujin suggested, "...get someone else with a newer Mac to DL it for you...".