What is the best way to get the biochemicals responsible for penile growth after puberty has ended?

None exist: Adult penile size is determined by a complex interplay between genetics and your hormones. When puberty is finished, you have what you were destined to have. If you never really finished puberty & are hormone deficient, it may help. Prescription injections under guidance of a specialist would be needed. If you are unhappy with your outcome, you can have more added by a urological surgeon.