live in the US? lowe's or home depot.
Live in Canada? home depot (usually doesn't have it) or a carpet store, yes they exist and yes they will more than likely have astro turf, want to know where your closest carpet store is? Use yellow pages. Now what you are looking for is "outdoor carpet" (be careful because some outdoor carpets are made with fibers like a normal carpet). What you want basically has little pieces of plastic put into carpet form. Go to a carpet store and feel the carpet, it should be and feel like plastic. It usually looks like this:
But isn't as high or as thick when it comes to the strands of plastic. Unfortunately not that many high quality pictures exist and I am not at home to take one at the moment, it also comes in various colours like black, blue, red, so don't worry if it isn't green.