Disregard this if possible: A shop somewhere with poison for sale, or make poison somehow craftable by assassins.
Also, increase the duration of poison on weapons so they're actually useable.
There is a shop that sells poisons...
If you have at least 1 level of rogue. Otherwise, he won't talk to you.
We have lots of fun stuff in the apothecary! And plan on adding more! *shifty eyes*
He'll talk to you if you know thieves' cant, I believe. It's been ages since I looked at that script.
Well, then, off you trot!

Well, then, off you trot!

Not sure if that's it. He just glowers at Anisa and she's got a few rogue levels and Thieves Cant.