Usually people get rubble from taking chunks of live rock, wrapping a towel around it and busting it up with a hammer or rubber mallet. Sometimes when you get rock, small pieces will break off during shipping/transportation. You could go to your lfs
and ask if they can sell you some if the have any, or you can take base rock and bust it with a hammer or rubber mallet as stated above but remember to wrap the towel around it to keep it from flying everywhere and hitting you. The base rock rubble will eventually become live rock rubble.