I came across this the other day, when the IIS install failed because 'the source files could not be found' on Windows Server 2016.
You need to locate your source DVD or, in my case, I had the ISO image on the server available to me.
Double click on the ISO image and copy the SOURCE folder to a temporary drive.
In the IIS install, select 'specify an alternate source path'. Point to ....\source\sxs from the copied source folder.
The install should now complete successfully.