Where do I order cyanide, arsenic, etc?
I don't think I'm smart enough to make any at home.
And plants are fucking trash at being stealthy poisons or for suicide purposes. At most they're interesting to discuss with what you can combine them and if there's a way to remove all the side effects like stomach aches.

Also where do I order much nastier stuff like organic mercury, thallium, uraniun?(or a way to oxidize the mercury I currently have)

Also I got an idea about how to release said substances in the air; perfumed sticks. I just put the substance in a jar and let the sticks infuse themselves with the liquid chemical. Just like night flowers you aren't suppose to keep while you sleep.