There are several ways of downloading the source code of Zabbix. You can get it from a Subversion (SVN) repository, which will be discussed in Appendix B, Being part of the Community; however, for this installation procedure, I suggest you download version 4.0.0 from the Zabbix home page, https://www.zabbix.com/. While it should be possible to use the latest stable version, using 4.0.0 will allow you to follow instructions more closely. Go to the Download section and grab the compressed source package. Usually, only the latest stable version is available on the download page, so you might have to browse the source archives, but do not take a development or beta version, which might be available.

To make further references easy, I suggested you choose a directory to work in, for example, ~/zabbix (~ being your home directory). Download the archive into this directory.