Chamomile tea can be beneficial for mild anxiety relief, as can Valerian, although that can generally make people very sleepy so more often used at night and to aid in sleep. Calms-forte contains multiple supplements that are generally safe, I can't speak to effectiveness personally or with research, but I've had some people tell me that it's worked.

Fish oils are backed with some positive research and are in general good for your health in a multitude of ways, so I'm a big proponent of them. This is a daily supplement though, not "as needed" and it takes awhile to kick in. You'd want to get a relatively high dosage for mental health benefit and in particular a ratio of more EPA than DHA.

Sam-E has some evidence supporting its use for mild depressive symptoms, but is expensive. 5-HTP is a newer one that's being touted and I don't believe that there is significant data in favor of it, though.

It's also worth checking in with your doctor and making sure you're not deficient in any vitamins or nutrients or that there's no other physical cause to explain it.

Regular exercise and meditation have good supporting evidence to benefit depressive/anxiety symptoms so you could consider those instead of or in addition to the above.

Good luck! Keep us updated on what you decide to try!