My Handi 10 gauge turkey gun has a steel rod inside the stock so you maybe able to get one from NEF if you call and ask, or you may find something suitable at a hardware store. The problem was mine rattled around inside the stock. So, I took in out and wrapped duct tape around the rod till it was a tight fit inside the stock. I need to pull it again because it has loosened a bit, more tape is needed.
The BB idea is a good one, lead shot would work as well.