Download Mountain Lion on unsupported Mac?
You should be able to use file linked here to download mavericks, just use "Disk Utility" to restore the ".dmg" to a usb thumb drive then boot your mac off it, then select your hard drive and then let it finish booting.

I used this to download Mavericks on a late 2006 model Mac mini with 2GB of RAM. Mavericks won't install or run on it, but I was able to use the downloaded app along with tools from the "tonymacx86.com" website to make a USB installer to install on a PC to make a customac (aka a hackintosh)

I wish they still had Mountain Lion available to download, If they still did I think this method would have also worked to download it too.