I've been pulled up for copyright on dA as well. I try to respect the rights of the models and photographers so I stick to using stock and follow the rules provided. Usually you just need to provide the models/photographers with credit in the description, however some restrict you to only posting your work on dA (which is why a lot of my stuff doesn't end up in other galleries). Occasionally, I come across photos that I'm inspired to work with, but aren't necessarily stock photos. In these cases, I ask for permission first. I've only ever been denied twice. Most girls are more than happy to let you use their photos and are always curious to see what you can do.
Finding the sources of your images is a lot easier these days with tools like
http://www.tineye.com and the ability to upload photos into Google image search (go into Google, click on images, then click on the camera icon to upload your own photo to search). These tools also make it easy for people to find potential copyright violations too though.
I'm personally not someone that cares much if my photomanips are copied and distributed, however I can see why people get up in arms about it. Whilst you may be creating an original result, you're still bound by the same copyright laws and rules as every other creative form (e.g. remixing a song without getting permission to use the samples first is comparable to using uncredited photos that you don't have permission to use). Its essentially art theft - even if there is an added original component to it. Many photo manipulation groups in DA won't accept your work if you don't provide links to all of your sources.