Has anyone used LEDs in their costume or another craft? I’m helping my partner make a Finn cosplay from Child of Light.

http://i2.cdnds.net/13/37/618x270/gaming-child-of-light-screenshot-1.jpg (on the right)

We have the hat and we need to add the light. I KNOW LED kits exist… I just want one LED wired to a battery that I can finagle into working. (Having an on/off switch would be good too.) But I dunno what to search for, because I’m not turning anything up. So if anyone has any ideas, let me know please!