You can't OD on OTC meds, that's why they are OTC. but I don't know were do you live, in my country codeine phosphate is easy to get because any doctor would give you prescription as it is sold as "cough pills", and contain no other active components.
you should know though, you should really take a lot and drink a bottle of ethanol 70% or any alcoholic drink with high alcohol precents, and if you have any other drug that suppress the lungs you should use it. you should do it when no one will be there for long time and no one would try to contact you, so by the time someone notices, you'll be dead.
you need at least 1200mg of codeine for an average adult to even have a chance, and alcohol to higher the chance.
13582 is right, the best way to get a really deadly stuff in silkroad3 on darknet, but they only take bitcoins.
but hanging youself is painful unless you do it from a high place and break your neck.
another method is explosives, TATP is suitable.