Natural Resources Canada has a browseable, searchable map that lists air photos going back to 1920. Coverage is very spotty and images of the photos are, unfortunately, not provided online but the site is potentially a great source for locating old air photos. You must register to be able to access the online map but registration is free. Air photos that meet the search criteria are listed along with their metadata. The site only seems to work with IE6 or Netscape 7.1. Prices for air photos start at $14.99.