Check out this web site for free coloring pages. There is a section for art therapy for anti-stress. There are other adult pages under various categories like Zen, Egyptian, Return to Childhood, Japanese and Tattoos.
Here are a few pages from the site so you can see what they are like. I have been getting into coloring myself and I find it very relaxing. Certain kinds of designs and pictures appeal to different people so choose what you feel good looking at. Do not worry about the category or if it says it is specifically for anti-stress. What works for you personally is what is best for you.