you should probably accept the fact that most deaths are not clean, quick and painless. there is a lot of suffering involved in the process of dying unless you do it in very specific ways, and those usually involve very hard to acquire drugs or messy methods like jumping, gunshot or a train, and there is always the possibility of something going wrong and having to experience pain even in the methods that seem instantaneous or pain free.
and you ask for one that doesn't require to buy anything... that only leaves hanging or jumping, and hanging can involve quite a bit of pain, not to mention that jumping takes a lot of courage and you need a really high place if you don't want to become one of those cases of survivors that end up paralyzed or extremely disabled.
please don't try anything impulsive, do your research and take some time before you attempt anything. don't take this as me trying to stop you or anything, you have the right to make this decision, but impulsive attempts are likely to fail and make you hate this life even more.