I have a relative who is an illustrator. She started doing labels for Fat Tire ales, then other people saw those labels, contacted Fat Tire, and got her name. Now she does graphic art for a few other companies, as well as a cpuple of children's books from people who saw the other labels and liked her style.
So, look around for artwork that fits your idea of the book - look in other children's books, or labels, or advertisements for a style that matches your text, then find the illustrator.