Where can I find a replacement power booster for our 65 dart?

You can go to a later model booster to replace your current booster AND vacuum storage tank, Which is your squarish box on top of the driver's inner fender connected to the booster with a hose. I am using this booster in my 65 Dart wagon. I had it rebuilt by "Booster Dewey"image.jpeg. My wagon had the same factory booster/storage tank as you do.