If you can't find such files on prep.ai.mit.edu, have a look at archive.cis.ohio-state.edu. Text mode does not work for tar files or compressed files. These are made with diff -rc2. The Emacs manual in source form is included in the distribution. GNU software - user anonymous, pass guest.
Some programs have misc support files as well.
Have a look on prep to see which ones. More information about these programs can typically be found in the GNU Bulletin. We don't distribute TeX now, but it is free software. TeX is a document formatter that is used, among other things, by the FSF for all its documentation. You will need it if you want to make printed manuals.