I don't know of any food for stress relief, but there are a lot of supplements. I used one by Homeopet called Anxiety Relief. There's also something called Rescue Remedy. Both are drops that you can add to wet food or water. I wasn't impressed with the RR. It didn't seem to have any effect at all. The Anxiety Relief may have, but it's hard to say, because I had a Feliway diffuser too.
There are also a lot of treats that are supposed to be for stress relief. I've never tried them. My vet gave me a NurtureCalm pheromone collar. I couldn't get it on Celia; the collar didn't fasten easily, so I needed two hands, which left me none to hold on to the wriggly cat. However, my vet said that of the thirty or so people who'd tried it, only one found it ineffective.