Mac Pro 2008 OS X 10.11 El Capitan

Hey,

i got a Mac Pro A1186 EMC 2180 2,8GHz 8Core 2x1GB RAM etc.

The former owner resetted the Mac completely and wiped everythig on the HDD out.

Now my problem is:

Re-installing OS X 10.11 via Apple Store doesn't work, as it's not longer available.

I tried to download the DMG and deploy the Mac via USB-Boot, but that didn't work too, because it told me the files got corrupted oder damaged.

I need OS X 10.11 El Capitan as Operating System.

Can somebody help me to deploy this Computer ?