Seek out a "Poison Master *" NPC in your home town, or whatever town. A short dialog and they'll offer a task. The task varies by level and number of tasks completed and if it requires any killing, the mobs level adjusts to suit yours. Mostly they require using Sneak / Hide to manuever aroudn East or West Freeport, sometimes planting a bomb, sometimes killing an NPC, that will be a weak dark blue con, and very easy to solo with defiant gear.
What you get is 20 each of the level appropriate Shissar poison, and the ingredients to make the poison, if you have the skill. There is a 7-day lockout on requesting a new task once you complete one.
At very low levels, <15, if I recall correctly, you don't get any poison, and only a few ingredients. Invis potions work instead of Sneak / Hide at very low levels. Being Invis or Sneak / Hide is very important, as the NPCs that hate Rogues and are involved in these quests are 99 level and hit for 96K.
That's about it.
Here's some of the Poison Master NPCs locations:
Rivervale, -30.10, -182.54, 14, Poison Master Elmn
The Greater Faydark, -524.71, -166.51, 160, Poison Master Tom
North Kaladim, 550.31, 213.17,- 32, Poison Master Feilicks
Neriak - 3rd Gate, 665.75, -1348.93, , Poison Master Oskar
Halas, 300.63, 177.04, , Poison Master Opto
Ak'Anon, 2072.60, -491.44, , Poison Master Genk
The City of Shar Vahl, 192.34, -342.90, -245, Poison Master Ajik
North Qeynos, 46.49, 188.23, -29, Poison Master Tenk
East Freeport, -1442.75, 347.19, -47, Poison Master Telkos