Supplements

That's a tough one, it'll be difficult to find a supplement that will have a dramatic effect on stress that's scientifically proven and not just placebo.

The best I can recommend off the top of my head would be Vitamin C, which has been shown to lower cortisol levels (stress hormone). However, this will only have so much effect on your overall stress, especially if the source of your stress is not resolved.

Being deficient in Magnesium, which is extremely common due to a lack of common dietary sources of magnesium, will also make you more susceptible to stress. A cheap 500mg Magnesium Oxide or Hydroxide supplement taken with meals will take care of that. And don't bother with more expensive complexed forms of magnesium, they have less elemental magnesium than oxide, and their quick absorption is not beneficial, as the human body can only effectively absorb magnesium at a slower rate.

I'd also recommend getting a blood test done to see what your Vitamin D levels are. Vitamin D deficiency can really fuck you up mentally, giving you dark thoughts, depression, and make your stress seem even worse.