The PostgreSQL 7.2.8 sources can be obtained by anonymous FTP from ftp://ftp.postgresql.org/pub/postgresql-7.2.8.tar.gz. Use a mirror if possible. After you have obtained the file, unpack it:
gunzip postgresql-7.2.8.tar.gz tar xf postgresql-7.2.8.tar
This will create a directory postgresql-7.2.8 under the current directory with the PostgreSQL sources. Change into that directory for the rest of the installation procedure.