The PostgreSQL 8.0.0beta5 sources can be obtained by anonymous FTP from ftp://ftp.postgresql.org/pub/source/v8.0.0beta5/postgresql-8.0.0beta5.tar.gz. Use a mirror if possible. After you have obtained the file, unpack it:
gunzip postgresql-8.0.0beta5.tar.gz tar xf postgresql-8.0.0beta5.tar
This will create a directory postgresql-8.0.0beta5 under the current directory with the PostgreSQL sources. Change into that directory for the rest of the installation procedure.