This will appear in the next issue of Full Circle Magazine:

Q The computer I use doesn't have an internet connection and I browse the web
on another PC elsewhere. How can I download Ubuntu packages so that I can save
them on my USB flash drive and take them back to install them on the offline PC?

A Go to the web site "packages.ubuntu.com" then select your version, and then
the application you want.

When an application needs something else to be installed (dependancy), the web
site will show you. It can be handy to have a list of what is already installed
on your computer, created by:
dpkg --get-selections "*" > Desktop/apps.txt
Then copy the file "apps.txt" onto your flash drive. That way you can see
whether you actually need any dependancy files. You have to be careful to
install the dependancy before the application.