Hi:
I found the page with postgresql-8.1 on it:
https://launchpad.net/ubuntu/+source....1.2-1~breezy1
postgresql-8.1_8.1.2-1~breezy1.dsc 874 bytes 34eb2abab7e82ae499309e64ac07e549
postgresql-8.1_8.1.2.orig.tar.gz 10.8 MiB 91d75231cfe329d788675cd252d3a195
postgresql-8.1_8.1.2-1~breezy1.diff.gz 20.7 KiB 35ca7244d9807978768527b749956eac
Here's the Official Ubuntu Community Documentation on this:
https://help.ubuntu.com/community/PostgreSQL
Also, check in your Software Manager or Synaptic Package Manager. That way if this package has other dependiencies Synaptic will know about it; inform you of them and install along with your package.
HTH