If you try to download macOS Server from the App Store it should download the latest available version for the OS you are running?
The latest version of OS X Server is 5.2 , and not compatible with Yosemite.
Since it is impossible to upgrade a Macpro 1,1 more than Yosemite, it is impossible to install OS X Server 5.2
So i've called Apple Support to get the version 4.1.5, but they told me that they didn't kept any archive of this app, and that i'll easily find it on third party website.
But , it seem all those website are fake or full of trojan and malware, thats the reason i'm asking here if anyone have kept this app in their personnal archive.