In addition to the commercial sources listed on this site, they can be obtained from the federal government (USDA, USGS, NOAA) and state, county and local government sources. Transportation, Assessors and Planning Departments are the most favorable sources.
Possibly, but it is not recommended that anyone try it. The original, un-modified negative probably exists in someone's archive.
Visible detail is a function of photograph scale and image quality, however vehicles are visible at a scale of 1″ = 1000′ and utility poles, fire hydrants and manhole covers should be visible at a scale of 1″ = 500′.
This is the ability to use a device called a stereoscope to view two photographs that show the same features taken from two different view points. The result is an apparent three dimensional model.