I know each has their own choices and reasons in life but please research some better methods.
Here in the UK we are very lucky, we have almost all methods easily accessible except firearms.
Poisons will usually give an agonising, slow death. Even something like potassium cyanide takes several minutes.
I think OP probably meant just a substance that could kill someone, but yeah you're definitely right in saying that a poison made to kill organisms and just for that purpose is
not the way you want to go out. All of them are painful in a way most, if not all, people will never understand.
For context, cyanide kills you by inhibiting cytochrome c (oxidase), which is used in the respiration of the cell by your mitochondria, so when you take cyanide to CTB, you're literally stopping all of the cells afflicted from producing ATP. Every single one will die off. That is
agonising, as you say. I'd wager it's as bad, if not worse, than self-immolation. Not a way you want to be going out. You'll also be convulsing, having seizures, immense chest pain as your heart gives out...it's not a fun time.
To actually answer OP's question though rather than my first lackluster response I gave above, if you really
do want to CTB, and you want to do it in style (painlessly), go grab opiods from somewhere. Prescription, DNM, just one that's potent. Then grab a benzo of whatever type you want, maybe an extra medication or two you find in the cupboard that depresses CNS and respiratory system, and some alcohol to potentiate the effects of your given opiod, and you will
probably die, depending on how much you take. This is by far the best method out there. It's better than SN because even SN causes some discomfort, and you'll probably vomit a bit or have a raging headache before you kick the bucket. Going by opiod though is painless. You literally just get really fucking high and then stop breathing and pass out without realising (depending on how off-your-head you are). It's the best way to go short of going under anaesthetic.