The PostgreSQL source code for released versions can be obtained from the download section of our website: https://www.postgresql.org/ftp/source/. Download the postgresql-version.tar.gz or postgresql-version.tar.bz2 file you're interested in, then unpack it:
This will create a directory postgresql-version under the current directory with the PostgreSQL sources. Change into that directory for the rest of the installation procedure.
Alternatively, you can use the Git version control system; see Section I.1 for more information.