Hi,
as IOhannes pointed out, all externals from Pd extended are mirrored on
https://git.puredata.info/.
Note that most of those mirrors contain the original code and are not
actively maintained.
Some people actually took over maintainance of individual Pd externals
on GitHub, e.g. https://github.com/electrickery
In some cases, the original author has the code on GitHub, e.g.
https://github.com/MetaluNet/moonlib