Picture from Superior Hobbies, Casselberry, FL.
Picture from HobbyTown USA, Concord, CA.
Rock Trail (real rock)
Picture from Remote Control Hobbies, Aurora, CO.
Rock Boards (foam & liner)
Picture from Jake’s Performance Hobbies, Rohnert Park, CA. & Axial Racing.
Rocks can be found anywhere and could be bought at an inexpensive price. You can even ask some of your customers if they can get some for you. If you cannot get rocks, Axial shows you how to make some artificial ones. Here is a link: http://www.axialracing.com/ftp/rockcourse/. For little to no cost, these courses are a lot of fun and will entertain a crowd of people in your store.
-James