Lizard Nobles are in Razachai (in Zao near Farmine) upstairs in various locations of what's also known as Lizard City. Killing 6 Lizard Nobles is apart of the Wrath of the Emperor quest mission 7 A Noble Cause. This mission can be seen in various Youtube videos, for example- https://www.youtube.com/watch?v=yAb5-zBa8AQ&t=9s which will show you the way to the Lizard Nobles.
I made a map below showing you the entire spawn.
The black line represents where you start. First of course ask NPC Zlak for a mission and making sure mission 7 appears in your Quest Log. You'll then follow the black line past the doors to the other side which is to what's called Lizard City which has Lizard Zaoguns, High guards, or Dragon Priests. Please proceed with caution if you're a low level, bring a friend or a team if you've never been here. The black circles on the map above mean Lizard Noble easy spawns I recommend. Please note, you will block the Lizard Noble spawn if you try to AFK there. You can, however, wait below the Lizard Noble at the spawns I recommend while you wait for him to spawn. For me, it took just a minute for another Lizard Noble to spawn that way you don't need to run from spawn to spawn and waste supplies. Beware, some Lizard Noble spawns have other creatures in some areas as well.
Also, on my map, there are some tags which represent the following-
Green arrow ^ = 2 Lizard Nobles
Red arrow ^ = 1 Lizard Noble
Red x = no Lizard Noble