If you have Bryce4, you can export your own rocks. 1. Create a rock by clicking its icon in the upper left of the window. 2. Then click on the small 'E' next to the rock. 3. When the Smooth Mesh dialog box opens, hold down CTRL and tap the 'd' key. That will allow you to export the rock as an obj. If you want to mess around, you can reshape the rock and change its material before you take Step 2.