I would agree with John in Kansas that USGS probably provides the best quality aerial photography readily available in stereo pairs. However, you might luck out with GoogleEarth, depending on the level of resolution available for your area of interest.
Another question is whether you are interested in photography showing changes over time, or just coverage that is most current. USGS is probably the bet source for that.
One of the nasty tricks we used to play on geology students in aerial photography lab was to reverse the stereo pairs in the set-ups and ask them to explain what they viewed. When the stereo pairs are displayed normal what you see is a three-dimensional landscape that gives you the perspective of flying over the area. However, when the stereo pairs are reversed what you view is an alien landscape where rivers appear to run along the tops of ridges and hills appear to be crators. Our students, those who trusted their eyes, came up with all sorts of creative explanations. ;~)