There are a number of products out there each with its advantages.
Polycarbonate, sometimes sold under the trade name "Lexan," is probably the most shatter resistant. Like the other sheet plastics it can be cut with a saw. It is clearer than glass, and usually used in 1/8" to 1/4" thicknesses. The main disadvantage is that it scratches easily and particular care must be taken while cleaning. No plastic glazing will last as long as glass though the better UV stabilized products will last many years.
Acrylic panels are not as strong but are cheaper and more scratch resistant.
You can get either from a glass place or from a place that sells plastic sheets, rods, and tubes. The plastic places are usually cheaper. Most of them will cut to size for a fee.