What about the `Source set directories'? I didn't find it nor in the
sparc64, neither in the amd64 ISO.
In this case, the installer (obviously, when you selected some source set to
be installed in a Custom installation) looks for
/<default_mount_directory>/<Base directory>/<Source set
directory>/<source_set.tgz>
There is one set of sources that are used for ALL of the various ports.
Look for ./source/sets/{gnusrc,sharesrc,src,syssrc,xsrc}.tgz
I've typically installed them under /usr/src, et al. (though usually symlinked
to a separate partition mounted as /Sources).
E.g., my /Sources contains:
./src
./xsrc
./pkgsrc
...