Dangerous
G wonders (somewhat bizarrely) where to begin inquiring about where people go to find poisons for the purpose of suicide. "I would appreciate some suggestions for the heroine of a story I'm planning to write."
Sorry G, but this is not explicit information any reputable publication would be willing to print for -- excuse the pun -- mass consumption. Far too dangerous.
Try a medical library. I don't know of one specifically but maybe the Tokyo Metropolitan Medical Information Centre on (03) 5285-8181, or AMDA on (03) 5285-8088 could help point you in the right direction.