If you can spare the disk space, build an archive mirror with debmirror
on an internet-connected machine, then move it to the non-connected network.
You'll need about 70GB of space for a full mirror of Precise (12.04) only.
A full ubuntu mirror (all versions, releases, CDs etc) is about 700GB.
You'll need to do some other stuff too, but there's really good documentation on the ubuntu site.