If you truly are having trouble finding a model, here are some suggestions.
1) As someone suggested ask around in your area. I know it is awkward having to ask a stranger, but it can actually work in your favor when you do the demo interview because it will be more real interacting with someone you don't know.
2) Communicate to the manager that you are struggling to find a model. Sometimes the manager has some customers that she knows love getting their makeup done who would be willing to be the model, or an employee who might be willing to help out. My manager found a freelancer to help me out for certification since I was struggling finding a model.