Some sites with information about obtaining and working with aerial photographs:
USGS Terraserver (US data)
USGS list of sources of old aerial photographs
LandVoyage aerial photo suppliers (commercial site)
MapMart aerial photography suppliers (commercial site)
USDA aerial photography field office
Getmapping: Aerial photos of the UK (commercial site)
Software for Processing Aerial Photos
ASPRS reviews of software for orthorectifying