After posting this I realized you wanted to get the prereqinstaller to do the download for you. However if you download them once yourself you can simply store the 6 files in your own directory.
just went thru this myself last week- I found this on the net somewhere and it worked:
1.) Place the downloaded files into a directory. For my example: C:\sp
2.) Run the following command via command prompt within the root directory of the install media:
PreRequisiteInstaller.exe /SQLnCli:C:\sp\sqlncli.msi /ChartControl:C:\sp\MSChart.exe
the prereqs are:
sqlclient,powershell, syncframework, mschart, geneva framework, adomd.net