Assuming a distribution like Ubuntu (Xubuntu 13.04, raring here), the synaptic package is part of the universe section ("Community-mainained free and open-source software") which may not be enabled by default in your case. So you may need to enable access to that source like so described here for instance. First, if you're doing this with a terminal, you may check the /etc/apt/sources.list file to see if you find any reference to "universe" and if not you may need to modify or add some entries. For example, in my sources.list file, I can find the following "universe" related entries:
deb http://ca.archive.ubuntu.com/ubuntu/ raring universe
deb-src http://ca.archive.ubuntu.com/ubuntu/ raring universe
deb http://ca.archive.ubuntu.com/ubuntu/ raring-updates universe
deb-src http://ca.archive.ubuntu.com/ubuntu/ raring-updates universe
deb http://security.ubuntu.com/ubuntu raring-security universe
deb-src http://security.ubuntu.com/ubuntu raring-security universe
Depending on your distribution and its release version, the entries may vary. You may try apt-cache with the
apt-cache search --full synaptic command to see any available package which you have access to and which references "synaptic" somehow. Finally, websites usually maintain search engines to allow you to figure out the contents of their repositories and which section a package belongs to and its dependencies, which in turn offers guidance for modifying the sources.list file.
Generally, see managing repositories from the command line with Ubuntu.