The PostgreSQL 13.1 sources can be obtained from the download section of our website: https://www.postgresql.org/download/. You should get a file named postgresql-13.1.tar.gz or postgresql-13.1.tar.bz2. After you have obtained the file, unpack it:

gunzip postgresql-13.1.tar.gz
tar xf postgresql-13.1.tar

(Use bunzip2 instead of gunzip if you have the .bz2 file. Also, note that most modern versions of tar can unpack compressed archives directly, so you don't really need the separate gunzip or bunzip2 step.) This will create a directory postgresql-13.1 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.