Reddit mentions of uxcell 3V Battery Holder Case Storage Box 2 x 1.5V AA Batteries Wired Pack of 5

Color changing fiber optic lights wired to a hidden battery pack in your back pocket. Or color changing LEDs. I know there are LEDs that you can get in a multitude of colors or even selectable ones, I can't imagine that fiber optics would be difficult to find as well. I'll try to find some links.

Edit: Okay, so here's what we've got.

• [Your fiber optic strands]
  (http://www.amazon.com/PLASTRUCT-92505-Fibre-Optics-PLSU2505/dp/B0006O6X8S/ref=zg_bs_5486433011_6)
  
  
• Your LEDs
  
  
• Your power source
  
  You'll likely need some electrical tape and maybe a soldering iron, and some length of wire. I'm on my phone so forgive me for not knowing ahead of time, but you'll need to use the right type of battery: the LEDs come with 200 ohm resistors, so you'll need to match that up to an appropriate power source using V=IR.
  
  Maybe you'll also be able to find a hosting to hold together your cables and the LED(s) but if you can't then I think you can just tape them right up. If all goes well, take the fiber optic strands and feed some through the cape, maybe tape or adhere some strategically along the bottom so they poke out, and maybe let some dangle.
  
  Good luck if you give it a go!

If you have electronic stores near you you should be able to just grab a battery pack like this one:

http://www.amazon.com/1-5V-Battery-Holder-Black-Leads/dp/B00E0IXRUS/ref=sr_1_1?ie=UTF8&qid=1426098517&sr=8-1&keywords=battery+holder