Try making your own pads or finding presets and pitching them fairly low..
If you want a choir sound like in 103 degrees, you CAN use a formant filter on a pad to some effect.. other than experimentation you can always resort to sample packs.
If you're looking for some really dark, horror sound effects I would recommend to you some of the Hollywood Edge Sound Designer Toolkit sample packs. Top quality stuff, you could use those bad boys in a movie.
Mangling samples you already have can be a great way to create new sounds.
Really, when it comes to ambiance, the sky is the limit.. experiment, experiment, experiment, or if you can sound design well imagine what sort of a scene you want to paint with your ambiance - and then make/use those sounds to achieve your vision.
