I've been working with collage since I was 16, and have maintained at least a trickle of ongoing work even after I had less time and space for my creative pursuits. As a result this is the media I've worked with the longest, but really it's the idea behind collage that is my focus.
One of the oldest endless debates behind "fair use" and copyright affects collage. Taking bits and pieces from the world around you and creating something new, which you take credit for, can indeed be called stealing. However the assembly of almost random elements into a new entity, and more importantly the process of doing this, is the point. It is a very challenging medium where failure is a fine line between too much or too little manipulation of the source(s). Like everything the fine line will exist in different places for different people.
My paper collage work has never been commercially used even though the experience has carried over to my completely different digital Photoshop work. I earn no money from it but it does come from commercial sources like ads in fashion magazines, photo journals and even pornography. And here is another point: these works age better than my paintings, sculpture or other artworks because the longer they exist the more the commercial context of the images change. I find that a beautiful thing.
A huge amount of gratitude is in order for the printers, publishers and photographers of National Geographic, my largest source of high-quality printed paper for as long as I can remember. In addition to that I am obviously indebted to the creator of anything I've cut up and used. If there was a list, it would be a a mile long.