For the most part ambient BGs are constructed. You can use a recording from a decent field recorder like the DR-100, or, even better, use a stereo mic with a mid-priced recorder like the FR-2, HD-P2, PMD-661, etc. Once you have a nice long bed you put in the "details" that really bring the ambiance to life.
I am always curious why everyone wants to own all of their gear. I do audio post (and have plenty of $$$ tied up there), so instead of buying a $400 prosumer recorder I rent Sound Devices or similar quality gear for the few times a year I need to do field recordings.