Many forms of chemical poisons are freely available to buy, especially agricultural and household chemicals. Many of these are highly toxic. Statistics from the World Health Organisation1 showed that pesticide suicide was a popular method in Asian and Latin American countries.
However, information on minimum lethal doses are hard to come by. On the alt.suicide.methods site there were numerous reports of people trying various poisons and stating they were a) non-fatal, and b) extremely painful, in some cases resulting in permanent physical damage. The pain might become excruciating long before death comes. So anyone considering this method may want to consider doing thorough research on the effects of their chosen chemical.
V Ajdacic-Gross et al. Methods of suicide worldwide, Bulletin of the World Health Organization September 2008, 86 (9).