You can view here to find latest package:
http://repo.zabbix.com/zabbix/
See here for LifeCycle Policy, as they seem to post guides for installing packages that don't exist yet!
http://www.zabbix.com/life_cycle_and_release_policy
(When I first tried to install Zabbix, I was trying to use a URL from the guide that didn't work, and ended up getting an old version from Ubuntu repo) (3.4)
A Ubuntu Example:
http://repo.zabbix.com/zabbix/3.2/ubuntu/pool/main/z/zabbix-release/
You want the file "zabbix-release_#+{DISTRO}_all.deb"
Eg:
cat << 'EOL' >zabbix_agent_ubuntu14.sh
#!/bin/bash
#Zabbix Variables
zabbixserver=192.168.1.255
hostname={HOSTNAME}
#Latest Package URL
latest="http://repo.zabbix.com/zabbix/3.2/ubuntu/pool/main/z/zabbix-release/zabbix-release_3.2-1+trusty_all.deb"
#Install Zabbix Agent + Server + Pre-Reqs
wget $latest
dpkg -i zabbix-release*.deb
apt-get update
apt-get install -y zabbix-agent
sed -i "s/127.0.0.1/$zabbixserver/g" /etc/zabbix/zabbix_agentd.conf
sed -i 's,^\(Hostname[ ]*=\).*,\1'$hostname',g' /etc/zabbix/zabbix_agentd.conf
service zabbix-agent restart
EOL
chmod +x zabbix_agent_ubuntu14.sh
./zabbix_agent_ubuntu14.sh