It also depends where you live. I think in bicycle meccas such as the Northeast or Northwest of the US you can find used ones. I live in North Texas where cargo bikes are as rare as hen's teeth. I have never seen any in bike shops in my area and used ones hardly ever show up either. Maybe once or twice a year. People here don't use bikes to run errands.
You may need to do nationwide search but then shipping can become expensive. In addition many private sellers are not willing to go through the hassle of packing and shipping a bike.