initdb is part of the
postgresql-9.5 package as provided by pgdg, and it should be located where you don't find it.
$ dpkg -S initdb
It's not normal if you don't find
initdb in the directory where it should be, so you might want to double check that, and maybe consider reinstalling the package.
initdb is not meant to be called directly in Debian, which is why it's not in the PATH. Debian packages for postgres come with an abstraction layer on top of all these commands, to allow for several versions of postgres to run simultaneously.
There is an introduction here: https://wiki.debian.org/PostgreSql and details in the manpages for pg_wrapper, pg_ctlcluster, pg_createcluster, ... and a few other pg_* commands.
Also, a default cluster is automatically created when the main postgresql package is installed, so you normally don't even have to create one, and a default database is accessible just after the install by running
psql under the