Hi Rowefx - I hope this is not too late but I've printed many 45ft stage backdrops and have learned a few things in that process. Dresden is right about the 96dpi resolution. Depending on where your banner will be displayed you can adjust the resolution. There is this simple math that you can use. Obviously you cannot, well you can, but don't have to, design at actual size. So you want to create your file that is relative in size.
So 9 feet is 108 inches. In a perfect world that would be 300dpi of 108 inches.
If you design half that size - 108/2 = 54 inches, you double your resolution - 600dpi. When you blow this up to 108 inches (double your canvas size) you would be at 300dpi (your resolution will be halved).
Half those 54 inches to 27 inches - a more manageable width to design at, with a DPI of 1200. So when you double 27 inches to 54, your resolution will be 600. At 108 inches, you are back to 300.
So - with that math in mind, you can now adjust the end result DPI your banner requires. If the end result should be 108 inches with 96 DPI, then do the math for when your canvas is 27 inches.
I hope that helps. Also, talk with your printer - it is always good to have a quick chat and ask him all your questions.