Using a standard installation of Linux Mint 19 Cinnamon 64-bit, these packages are contained in the standard repositories. Performing the command you issued above on a mostly clean, unmodified LM19 installation yields the following results:
root@steve-VirtualBox:/home/steve# apt update && apt list | grep iperf
Hit:1 http://archive.canonical.com/ubuntu bionic InRelease
Ign:2 http://packages.linuxmint.com tara InRelease
Hit:3 http://security.ubuntu.com/ubuntu bionic-security InRelease
Hit:4 http://packages.linuxmint.com tara Release
Hit:5 http://ppa.launchpad.net/ichthyo/zeug/ubuntu bionic InRelease
Hit:6 http://archive.ubuntu.com/ubuntu bionic InRelease
Hit:7 http://archive.ubuntu.com/ubuntu bionic-updates InRelease
Ign:8 http://dl.google.com/linux/chrome/deb stable InRelease
Hit:10 http://archive.ubuntu.com/ubuntu bionic-backports InRelease
Hit:11 http://dl.google.com/linux/chrome/deb stable Release
Reading package lists... Done
Building dependency tree
Reading state information... Done
5 packages can be upgraded. Run 'apt list --upgradable' to see them.
WARNING: apt does not have a stable CLI interface. Use with caution in scripts.
iperf/bionic-updates 2.0.10+dfsg1-1ubuntu0.18.04.1 amd64
iperf3/bionic 3.1.3-1 amd64
libiperf-dev/bionic 3.1.3-1 amd64
libiperf0/bionic 3.1.3-1 amd64
root@steve-VirtualBox:/home/steve#
Note the difference here where the update portion is actually parsing and updating from the repositories, either you neglected to copy & paste this output into your question, or you appear to not have any repositories active.
I would suggest opening Update Manager in the GUI, and going to Edit - Software Sources, and on the Official repositories
tab, at the bottom, click Restore the default settings
and then close the window and select Refresh.
Software Manager application also shows iperf and iperf3 are in the repository and available for installation.