What is your aim and what is the question that you wish to answer with your research? I do not think that there is such a model or cell line that will allow you to study a disease like AD. Dementia and AD is arguably the most complex human disease we are facing with so many different biological forms. The next think available is the mice models but they also have significant issues again due to biological complexity and variability of the disease. So one think you can do is get one of these animal models and harvest cells (isolated or slices) from them. However be aware of limitations of such models in already over-crowded field.