On the advice of coffitivity, I've turned to human environments for ambient noise. I've been blessed with library access to the BBC sound effects library. http://www.sound-ideas.com/sound-effects/bbc-41-60-cds-sound... Some are the typical sfx fare, but the latter half is of city streets from around the world.
SoundTransit http://turbulence.org/soundtransit/index.html has interesting field recordings all under the CC BY 2.0 license. If you "Book a Transit," it compiles an mp3 of several different field recordings, taking you from city to city. You can also search the database or just browse the collection.