L-Trpytophan metabolizes into seretonin, things like turkey have this, but thats not an esay way to ingest it, not sure if you can buy it as a supplement.
But I think it first metabolizes into 5-HTP which you can also buy;
so you'd save your body a step by just buying 5HTP
you can buy it OTC but it can sometimes be pricey
Melatonin works as a sleep aid if you arre in need to reset your clock, like from a long flight ro something like that. For long term sleep problems its not your best bet but its relatively cheap.
I wouldn't use it for trying to boost seretonin, there has t be better things out there; namely 5-HTP