Where is data from Software Update server located?
The standard directory in Yosemite/Mavericks where the update packages are stored in folders and sub-folders is here:

"/Library/Server/Software Update/Data/html/content/downloads/"


The DocRoot may be customized though.

Entering the following in Terminal to get a previously customized DocRoot should work:  

 /usr/libexec/PlistBuddy -c 'print :updatesDocRoot' "/Library/Server/Software Update/Config/swupd.plist"