The downloads for Ubuntu 12.04 Precise Pangolin are available at the following links.
precise-desktop-i386.iso 12-Feb-2012 07:59 719M Desktop CD for PC (Intel x86) computers (standard download)
precise-desktop-powerpc.iso 12-Feb-2012 07:59 726M Desktop CD for Mac (PowerPC) and IBM-PPC (POWER5) computers (standard download)
precise-desktop-amd64.iso 12-Feb-2012 07:58 717M Desktop CD for 64-bit PC (AMD64) computers (standard download)
These ISO images are slightly oversized for a 700 MiB CDR, you will need a 1 Gigabyte mini DVD disc or a full 4.7 Gigabyte DVD-R to burn these images. There are no downloads available yet for the Ubuntu 13.04 distribution yet, that will come out when it is done.