Drug ODs are unreliable because the dose varies between individuals. Usually there will be a mix of different drugs if it should lead to a somewhat more predictable outcome of death. A drug dealer will never help you with suicide if you tell them that it is your intention so you will have to study yourself what drugs to buy and how to mix and administer them. Drug dealers only make money, and they may not know any lethal combination because most street drugs don't cause death that easily. Drug dealers risk prison time for drug dealing, they don't wanna risk doing prison time for manslaughter. To get someone else to help you die you would have to find someone who is willing to go to prison for life or is a doctor in a country that has legalized voluntary euthanasia. Sorry, it's not that easy.