This page of our guide to Hitman 2 contains tips on where to find lethal poison in Mumbai. This poison can be useful in eliminating mission targets. Information about opportunities for using poison can be in the walkthrough for Three-Headed Serpent mission.
The train yard, where Vanya Shah stays, is the first place where you can find lethal poison. The bottle is in Shah's wagon, the same one which you can access by, i.e. wearing the tailor's outfit. It is possible to encounter a servant in the wagon. Also, a man who pours Shah some tea stands near the rear entrance (this is a perfect opportunity to poison the target) - avoid getting detected by these two characters.
The Kashmirian's room is another place where you can get lethal poison (this room belongs to another paid assassin). The room is in chawls, the residential building near the default starting location. You visit this place to, i.e. hack into the Kashmirian's laptop. The bottle is near the suitcases.