Some publishers provide illustrators if they like your story. They'll match you up with an illustrator whose style fits yours. Otherwise you have two options: you can advertise for someone (locally and online) or you can ask around. You might make inquiries using social media, or you might find someone at your nearest college who is studying illustration. The trick is finding someone you can work with who will be willing to make changes as required. It can take time to find the right person, but it's worth it when you do.