I need to install LaTeX on a old MacBook holding a Mac OS Lion 10.7.5 . Apparently the new version of MacTeX distribution require Mac OS 10.10 or higher, and I am not managing to find old versions of the distribution. Some ideas?
You can simply install texlive directly (https://www.tug.org/texlive/acquire-netinstall.html). It support osx 10.6-10.9 with the
This will give you a complete and up to date tex distribution.
At this point you can stop reading this answer, as you have everything you need to build tex documents.
In comparision to mactex this solution only lacks some additional programs, for example a GUI to update your tex distribution (TeX Live Utility).
If you nevertheless want these program, you have two possibilities:
you could install an older version of mactex (see DG's answer for details). This will install old versions of these additional programs which work well alongside an up to date vanilla texlive installation
manually install current versions of the additional programs you desire. Suitable versions can be acquired from their websites (http://pages.uoregon.edu/koch/texshop/texshop.html, http://bibdesk.sourceforge.net/, http://amaxwell.github.io/tlutility/, http://excalibur.sourceforge.net/downloads.html,https://www.chachatelier.fr/latexit/latexit-downloads.php?lang=en)
There seem to be two possibilities:
Install an old version of MacTeX from the archive on this ftp server: ftp://ftp.tug.org/historic/systems/mactex
Install TexLive 2017 with the unix installer (which egreg managed to do). Instructions are in the answers to this question: New tex live or old MacTeX?
(I'd go with samcarter's answer, which is more elegant and straightforward)