One of the most common sources of info is the series of books published by Osprey:
http://www.ospreypublishing.com/world_war_2/
The Flames of War campaign books (river of heroes, d minus one, etc.) are fairly good sources for info on historical forces.
After picking one from those sources, I find googling the name of a specific regiment gives more detailed information on organisation and structure than can be found elsewhere, and you can normally find photos of the specific vehicles you are modeling as well provided it is not a really obscure force.