Just bumping this, as I think posting just the link may have been too little.
H2S is a very deadly gas that can kill you from a single breath. It's easy to make, potential ingredients are listed in the link. And it's painless.
The method became popular in Japan in 2008 when it was circulated on suicide forums over there, and they experienced a bit of a suicide wave there where even 12 year olds were managing to ctb.