Latest Zabbix 3.4.14 on https://repo.fredprod.com for Raspbian stretch.

Add repository

deb https//repo.fredprod.com/ stretch main

to a file in

/etc/apt/sources.list.d

Keyserver is: keyserver.ubuntu.com
Keyid: 1DFF230644068194

You can add gpg key:

apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 1DFF230644068194

Just run

apt-get update

After that, you can install or update packages for your Raspbian stetch.
This repository is only for armhf architecture.

You can find packages of zabbix for other architecture using zabbix mirror repository. Zabbix repository mirror for Debian & Ubuntu