How soft is soft in your opinion?
If you're just after pinups, for references, literally, look up pin ups on google for poses, or deviant art, except they've changed their search engine so its now a pain in the butt to get a good selection of nude poses but they were really good for me for finding nice poses and lighting references.
If soft to you, is a bit more sexual than just pin ups... honestly, you may just have to grin and bear it and google it, google image search generally is where I go if I'm looking for something that isn't just artistic nudity.
Primarily though, anything NSFW will greatly benefit from an understanding of anatomy, so life drawing, biology images, etc, will all help you as references so you can understand the anatomy of the body better. Most of us probably don't really know exactly how our own bits look as we can't exactly get down there to see! So it doesn't hurt to educate yourself on anatomy some too :3