Egg crates are found at hardware store under lighting or ceiling panels. Any of the big box store should have it. <eg HD, Lowes.>
You put the eggcrate down on the glass, then the rocks on top of that. It spread out the weight of the rocks & prevent sharp edges from scoring the glass. If your rocks are not stable, you might want to silicone them rather than just simple stacking ... or they might fall over & damage something.