On this page of the Hitman 2 game guide, you can gain information on where to find lethal poison in the Whittleton Creek area. This can help you with eliminating both of the mission targets. Information on good occasions for utilizing lethal poison has been included in the walkthrough description of the relevant mission on the previous pages of this guide.
The first location where you can obtain lethal poison is the Reservoir, which has been marked in the picture above. A rare species of frogs is being kept there. The area in question is located next to the park and the Cassidy Property. The reservoir zone is off-limits and it is worth keeping a low profile while trying to gain entry.
You must catch one of the Lethal Poisonous Frogs (you can see an example in the picture above). A frog can escape easily if you approach or run up to it. A better idea is to sneak up in the crouching position.
The second place where you can obtain lethal poison in the basement of the West Property, which has been shown in the picture above - it is a building in the north-western part of the map.
It is best to break into the basement through the side door because several people are inside the building. In the same basement, you can also find rat poison, which is helpful when you need to poison a target without causing its immediate death. This opens up new possibilities, e.g. of drowning Nolan or Janus in a toilet.