When you use apt-get/Synaptic etc to install a package the .deb is saved in /var/cache/apt/archives
The problem with trying to use a .deb on a system with no internet is you will run into dependency issues.
For offline installs take a look at:
Keryx Project*|*Updates for offline Linux users
Welcome to Sushi, huh? package downloader for offline GNU/Linux systems
Description: Update a non-networked computer using apt and removable media
These scripts simplify the process of using dselect and apt on a
non-networked Debian box, using removable media like ZIP floppies and
One generates a `fetch' script (supporting backends such as wget and
lftp, in a modular, extensible way) to be run on a host with better
connectivity, check space constraints of your removable media, and
then install the package on your Debian box.
Clone Your Ubuntu installation