I grew up during the time of doodle art - remember those? You got a pack of felt markers and elaborate black line drawings on a number of themes. I would spend hours coloring these in! Little did I know then that coloring pages were a benefit to my well-being.
Occasionally, you'll find official cartoon or TV character sites (such as PBS programs) will make available coloring pages for free to their fans. And there are many other sites that distribute coloring pages of those copyrighted characters. Sounds like dangerous legal ground ...but, surprisingly, I've never heard of any of these sites being prosecuted for copyright infringement. So they'll probably continue to make them available.
You don't have any difficulty to get the coloring pages for children -- you are living in an era of internet -- that has entirely revolutionized the data and information, which wasn't thought of even seven to eight decades before. A totally new dimension of coloring and drawing has been opened up with the arrival of the Net. But today one can find a large number of sites from which you can print out fine quality coloring sheets on just about any subject that you can imagine.