Are you saying that non-Parkinson's patients are taking L-DOPA as a supplement? If so, that's nuts.

Also dopamine is a precursor to noradrenaline, and serotonin levels are affected by numerous drugs, the most popular of which are antidepressants. That "supplements" for these neurotransmitters has more to do with how effectively they cross the blood-brain barrier and how they might disrupt the gut, since many chemicals in the brain do double duty through functions in both brain and gut.