My friend, "quick & painless" is the Holy Grail of suicide; if it was that easy, d'you really think discussion forums such as this would exist??
Pentobarbital is the aforementioned Holy Grail of suicide, but short of travelling to South America and buying it (or taking your chances and buying it online—which is VERY risky, as you don't know WHAT you're buying) "quick & painless" is not very easy to achieve and requires a A LOT of research in advance.
Like all of us on here, you have to do the work and find a method that might work best for you...