The PostgreSQL 9.0.20 sources can be obtained by anonymous FTP from ftp://ftp.postgresql.org/pub/source/v9.0.20/postgresql-9.0.20.tar.gz. Other download options can be found on our website: http://www.postgresql.org/download/. After you have obtained the file, unpack it:
gunzip postgresql-9.0.20.tar.gz tar xf postgresql-9.0.20.tar
This will create a directory postgresql-9.0.20 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 H.