Need 10.10.0 installer for Mac Pro

I have a 2008 3.1 8 core Mac Pro currently with 10.8.5 Mountain Lion which runs fine and I want to use as a second music slave computer . Some apps need Yosemite which is apparently the highest I go with this computer. I downloaded the 10.10.5 combo update but when I tried to in stall it it says I need a 10.10 install first . The problem is, I can not find 10.10.0 installer anywhere . Anyone know where to find this installer . Also is it possible to go directly from Mountain Lion 10.8 to Yosemite 10.10 ? Or do I need to install 10.9 first ?

thanks