In general it is easier to jump the bridge than to find a good poison if you are not skilled in procedures like extraction, distillation, chromatography, recrystallization and so on. Even then, extracting and purifying 1 mg is a challenge.
If you are just looking for information about strong toxins, some googling may help. Wikipedia is full of such information. I think 1 mg of ricin is well above lethal dose, in the case of botulinum it is even below 1 μg - but I can be wrong, as I am not even trying to check and my memory plays tricks now and again.
Hopefully you are just asking this question out of interest and you don't plan to do any harm to yourself.