I have has success with Valerian Root, it is cheap, you can get it at Wal-mart, it does not knock them out just takes the edge off. Start out with two and see how it works, you can increase it or decrease as needed.
Some examples of how I have used it when I got a new horse in who was running around really stressed about being in a new place I gave him four capsules, broke the powder into some feed within an hour he was quiet milling around his new stall. When one of my geldings got an injury to his face he would not let me look at it I gave him 5 valerian root and it about knocked him out I was able to clean him up while he was relaxed. Same gelding had to have surgery and was confined for 8 weeks, I gave him two a day, one in the morning and one at night and it kept him from going crazy in his stall during his recovery. I have another mare who gets worried about the vet I give her two an hour before the vet gets here and it just takes the edge off so she is more relaxed and cooperative. So you can experiment and adjust as needed, Valerian root is in a lot of calming supplements but is mixed in with other ingredients and they are pricey, like I said above it is cheap, readily available and easy to use.