There are a least these apt-get extension packages that can help:
apt-offline - offline apt package manager
apt-zip - Update a non-networked computer using apt and removable media
This is specifically for the case of wanting to download where you have network access but to install on another machine where you do not.
Otherwise, the --download-only option to apt-get is your friend:
-d, --download-only
Download only; package files are only retrieved, not unpacked or installed.
Configuration Item: APT::Get::Download-Only.
Try
apt-get -d install <packages>
It is documented in man apt-get.
Just for clarification; the downloaded packages are located in the apt package cache at
/var/cache/apt/archives
Don't forget the option "-o", which lets you download anywhere you want, although you have to create "archives", "lock" and "partial" first (the command prints what's needed).
apt-get install -d -o=dir::cache=/tmp whateveryouwant
Source: Stackoverflow.com