zabbix-agent package is only available in the
wheezy-backports repository, you should enable this repository if you want the software in wheezy (stable).
The package is also available in jessie (testing) and sid (unstable).
The procedure to enable the wheezy backports is adding:
deb http://YOURMIRROR.debian.org/debian wheezy-backports main
YOURMIRROR with your real mirror)
sources.list, then run
apt-get update to update your repositories then installing the packages using your preferred apt method.
sudo apt-get install zabbix-agent
Normally to install packages from backports you need to specify that you want them with the
-t switch, since all backports are deactivated by default (i.e. the packages are pinned to 100 by using
ButAutomaticUpgrades: yes in the Release files.), but in this case, there shouldn't be problems because is the only version of the package available.