https://sanctioned-suicide.net/threads/list-of-resources.3/ The recent editions of the Peaceful Pill Handbook discuss it in detail, including acquisition. They can be downloaded in the resources thread.
Nembutal was a popular sedative back in the day. It is a powerful nervous system depressant and very reliably lethal, unlike most modern pharmaceutical drugs. It is now used in end of life clinics and as a veterinary euthanasia drug. You take it with some anti-emetics (to guard against vomiting up the overdose) and rapidly lose consciousness (very deep sleep), enter a coma, then die of respiratory depression within hours. Usually 1-2, but it can sometimes be more. Barbiturate overdose is where a good deal of the myth of a peaceful, simple overdose on sleeping drugs came from, as people actually did overdose all the time (including accidental).
We have a megathread for discussion, especially pertaining to ordering: https://sanctioned-suicide.net/threads/megathread-getting-n-from-a.2544/