Exit International recommend Nembutal (which is also used by Dignitas). But it's the very devil to buy; dark-markets sometimes offer it, bit it's fabulously expensive, and the sellers never have a trading record.

I'm told that hanging yourself from a doorknob is fairly quick and quite painless, but it's a problem for anyone that discovers you, or is with you when you do it. And it's hard to be sure that some idiot won't try to resuscitate you.