If you ask at your local Overwaitea or other grocery chain, sometimes they'll have them, or even better a local butcher or fish shop. I also had a though that sometimes you get stereo or computer components in a similar weight of Styrofoam, so ask at the Radio Shack or other appliance store. Around here, you can't recycle them so they just go into the landfill
so they will be happy to have you take them. I'm going to do some research on if there's a kind of glue you could use to stick some separate pieces together - imagine pieces stacked to make it look like a rock outcropping!