Gravel of varying sizes and river rock: I've found them at landscape supplies, plant nurseries in rural areas, and the big box hardwares (Lowe's, etc.) have it at the back of the soil/mulch section. I also stopped once when I saw a big landscaping operation at a house in the neighborhood and asked him where I could get smooth gravel and rocks. "How much you need?" He asked, and gave me a bucket full. I tried to pay him, but "Nah, I get this stuff delivered by the truckload" was the reply.