Synaptic is available in the so-called universe repository. If you cannot find that package, it means that you do not have universe enabled. To enable it, follow these steps:
Open Software Sources and select Community-maintained free and open-source software (universe)
After doing so, you will be asked to reload the software sources and you will be ready to install your package.
Open the file
And add the following three lines:
deb http://archive.ubuntu.com/ubuntu/ quantal universe
deb http://archive.ubuntu.com/ubuntu/ quantal-updates universe
deb http://security.ubuntu.com/ubuntu/ quantal-security universe
Note: I'm assuming that you are using Quantal (12.10). If this is not the case, then use
lsb_release -c to find the codename of your Ubuntu release and replace
quantal with that codename.
sudo apt-get update and install Synaptic.