Many stores that sell grill parts might have just what you need in stock for under twenty dollars. GrillPro, one of the Onward Manufacturing Companies, makes a universal fit rock grate that fits most sizes of gas grills. Alternative to Lava Rocks. Of course, you don't have to use lava rocks. read more
The majority of modern grills use "tents" though I have seen some that still use lava rocks. Adding rocks will not do any good and will more likely cause problems or reduce the efficiency of your grill. read more
Very few grills use actual lava rocks anymore, because BBQers (yes, they're different from grillers, look it up) would use them until the porous lava rocks became completely saturated with sauce and fat, thus setting up for a conflagration that would melt the siding off your trailer. read more