I've never had to order more panels, so I am not sure if Chromo sells replacement panels, however, i am sure you can make a replacement panel for cheap. if it is the white panel, just get translucent or velum paper, its like $0.10 a sheet, cut it to size & slide it in the LED light or use tape or velcro. for color ones you can order warming gel sheets & do the same.
I would make my own out of milk bottle plastic or cut your own Rosco color correction gels. Velum is another great option and it's inexpensive, so you can make a bunch of them. The clear/white panel that is included does nothing to diffuse the light. It just protects the LED bulbs.