SOLVED: Installing .NET Framework 3.5 on Server 2012 Errors Out With Missing Source Files Error

The solution to “The source files could not be found” when installing .NET Framework 3.5 on Server 2012 (or Windows 8)  is to:

1. Pop in the DVD (or the ISO for the DVD)
2. Start the ADD ROLES AND FEATURES Wizard and click the obvious choices up until the CONFIRMATION screen
3. on the CONFIRMATION screen of the ADD ROLES AND FEATURES WIZARD, you need to click the SPECIFY SOURCE link (tiny link at the bottom of the window)
4. Point the source path to D:\sources\sxs\ (obviously change D:\ to whatever your DVD drive letter is)

Also note that if you download the .NET 3.5 installer from the web and then try to run it, the install will error out with:

Install from roles and features

For more detail read MS KB2734782:

In Windows 8 and in Windows Server 2012, the .Net Framework 3.5 is a Feature on Demand. The metadata for Features on Demand are included in Windows 8 and in Windows Server 2012. However, the binaries and other files associated with the feature are not included. When you enable the feature, Windows tries to contact Windows Update to download the missing information to install the feature.