The PostgreSQL 12.1 sources can be obtained from the
   download section of our
   website: https://www.postgresql.org/download/.  You
   should get a file named postgresql-12.1.tar.gz
   or postgresql-12.1.tar.bz2. After
   you have obtained the file, unpack it:
gunzip postgresql-12.1.tar.gztar xf postgresql-12.1.tar
   (Use bunzip2 instead of gunzip if you
   have the .bz2 file.)
   This will create a directory
   postgresql-12.1 under the current directory
   with the PostgreSQL sources.
   Change into that directory for the rest
   of the installation procedure.
  
You can also get the source directly from the version control repository, see Appendix I.