Install specific older version of postgresql centos I need to rebuild a production system with same version of installed packages.

It's a Centos 5.11 and it was using this repo http://yum.pgsqlrpms.org/reporpms/8.3/pgdg-centos-8.3-6.noarch.rpm to install postgresql-server postgresql postgresql-odbc postgresql-plperl postgresql-contrib and postgis.

That repo doesn't exist anymore so I found this http://yum.pgrpms.org/8.3/redhat/rhel-5-i386/pgdg-centos-8.3-7.noarch.rpm that's versioin 8.3-7 and I need 8.3-6.

I can't seem to find the old version I need on the official website yum.pgrpms.org/repopackages.php

I don't know if old mirrors for the version I want exists or if I need to download each package individually matching the version that was listed on that one repo.
You probably want to install from the PostgreSQL YUM Repository as it has replaced both of those repositories. The repo rpm for CentOS 5 i386 is here for download. 

The difference in the version on that particular package are from changes to the repository URL, and not changes to the versions of the related packages.

The last version of PostgreSQL 8.3 (8.3.23) and related packages are available there for use but that version and its packages are no longer supported.

If you need to build a very specific version of those packages outside of the last versions in that repository, you can always checkout the pgrpms git repository and rebuild from the .spec files there.