Sometimes I record stuff I hear in ads on the internet, like that sample I use in the second track on my last album [37 MB lol get the whole thing plx], or long speeches, etc.
The great thing about ambient sounds is that you can record whatever the fuck you want to record, then process it to death to make it fit your song as it changes. It all depends on what you want to hear right at that moment.
I'll tell you what's good for soundscapes? Check this out:
http://www.hgf-synthesizer.de/se/STS/STS.html