Commercial sites have archives which will cost you a lot less, if they will serve your purpose, than commissioning a fly-over for a detailed shot.
Commercial firms are equipped to do straight down shots, and can also geo-coordinate them digitally if you plan to integrate them into some geo-information system.
If you email large aero-photogrtaphy services and ask if they have something for (area) at (year) they can often provide the very thing. If it's an older analog file neg or copy they will scan it so you can have it digitally. Everything for a price, of course.