I do know that hemlock was used by Socrates, but on my world there are no hemlock trees. I guess the killer could have imported enough hemlock to make a poisonous drink, but then I'd have to explain how he/she did that. Too complicated.
I also know that foxglove is poisonous. However, the plant is native to Europe, western Asia, and North Africa on Earth. The real killer here (pardon the pun) is that while every part of the plant is toxic, it takes too long to kill its victim. Besides, it is not indigenous to the planet in the book.
Since those are the only two "exotic" poisons I know, it was time to stop writing and do a bit of research to find the perfect poison. Easier said than done I found. As I Googled poisons, I wondered if anyone at Google was keeping track of such things. Did I just get put on a list of some sort so that if someone ends up dying of poison in my town, the police will be at my door? Surely, not she said, with more bravado than confidence.
Next, I'm off to Amazon to see what books on poisons might be of assistance. I found one, Howdunit - Book of Poisons by Serita Stevens and Anne Bannon that looked interesting. A quick perusal of its table of contents confirmed that it was what I needed.
I hit my local library's website and looked up "poisons" in the catalogue. They have several books with the word poison in it, including my favorite, Poisons Make You Sick. Ya think? But they don't have any books on how to use poisons or what are the results of using poison.
Back to Amazon. I ordered the Stevens book and now the waiting begins. I'm only hoping that by the time the book arrives that I haven't changed my mind and have the character who started all this research die of natural causes or, worse, I decide to let him/her live.
I love writing!