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
