Check out tapplastics.com
They have a large variety of acrylic that you can get cut to size for some good prices.
If you are planning to do what they are doing just get transparent. It looks like they are using white paint (and blue for the sword stripes) but a very thin layer so the light comes through still.
Oh, and no, you need to "score" the plastic. Not use scissors. They won't work.
That site sells them. They are "plexyglass scoring tools"
Cosplay DA Regular DA
-Tsuruya-san (Haruhi Suzumiya)
-Kagami Hiiragi (Lucky Star)
-Minimandy (Grim Tales)
-Mimi (Grim Tales)
-Yui Hirasawa (K-On!)