This guide will let you how to get Khut-Khut Venom Sac in Warframe.
To get the Venom Sac, you will need to fish for Khut-Khut.
You can find the fish during the daytime; the first best IMO is shown in the image below.
You will need a fishing spear; all spears work on the fish.
Having a bait could be useful too.
There it is, each fish you one venom sac with every size. So, there is no need to find bigger fish.
The second best place IMO is shown on the map.
Keep in mind that you can only find the fish during the daytime.
To get the venom sac, you need to cut fish from the fishing NPC.