I have seen colored sugar crystals at my grocery store, in the baking section. They come in a small jar (the size of a spice jar) with a shaker top. At Christmastime, they're commonplace in red and green. They are used to decorate the top of cookies.
I have also seem them at a local craft store, Michaels, in the cookie/baking section, and a local cooking supply store, Sur La Table, in many different colors.
The activity calls for colored crystals because they will be easier to see than the standard white crystals from your sugar bowl. You can use plain white sugar, but you'll have to look harder at them to see them bounce. Alternatively, you could use colored plastic wrap (with white sugar), which is often available around holiday times. The point is to creat color contrast between the plastic wrap and the crystals, since the vibrations from your voice are pretty small.
In case you can't quite get the picture of the product, I've included a link below to the Sur La Table web site that has a photo of jars. You don't need to mail order this; your grocery store should have it.