So I just picked up one of the light-up Tron identity discs on Ebay.
Not super cheap, but they don't manufacture these anymore, and cosplayers are scrambling for them. I still got a pretty fair price compared to how much they are usually going for these days.
I went ahead and bought the black one.
Considering that I'll have to take it apart to bare bones and modify the lighting and wiring, I figured that I may as well paint the body pieces white in the process.
The more rare white versions, when I see them, are going for nearly $150 to $200. I bought the same model in black for $50. I can take the money I saved and literally pay for the light up EL Tape and EL Wire for the illuminated areas of the costume.
Another nifty detail; the little box below the disc on her back, is the battery pack for the lighting.
I remember seeing this in the film as well, and thinking, "awesome! no need to try and hide it further". Still, I may see if I can load it all into the disc base unit instead, for a sleeker look.