Install postgresql. Why is initdb unavailable?

You will find initdb under /usr/lib/postgresql/x.y/bin/. See also /usr/share/doc/postgresql-common/README.Debian.gz for more information on the setup on Debian and Ubuntu.

initdb is intended to be run under the postgres user account that is created during the install. After installing postgresql you can do:

sudo su - postgres

Then you should be able to run initdb.

initdb is not installed as user executable. Is only installed in /usr/lib/postgresql/X.X/bin/, because it always depends on the version. initdb can only be executed from that specific directory.

As mentioned in other answers, installation of postgres creates a default directory that may be in a limited partition. Users may want to change this, but it requires other steps also. see here.