Probably the easiest way to get a hold of pages for your kids to color is to print them out from websites online. There are plenty of sites that offer free downloadable coloring pages. Just search for "Christian coloring pages" or "Biblical coloring pages" in your favorite search engine, and you will find a bunch of results.
Local Christian bookstores usually also have individual pages or reproducible coloring books from which you can make copies so that more than one child can color in the same story picture. This is particularly useful if you want to give your Sunday School students something to color while you are teaching the lesson.