A big part of the magic of any children’s book is the artwork. But if you’re not Beatrix Potter, you’re going to need to collaborate with an illustrator. If you don’t have one lined up, it can be daunting to try and find one whose style fits with what’s in your mind’s eye and who is affordable and congenial.
Visit Fiverr and other sites such as guru, odesk, and elance, where freelancers hang out. There’s even ChildrensIlllustrators.com, as you knew there would be.
Make sure your illustrator has verifiable references, is willling to work under a written agreement (good ones will have their own), and that you have no lingering doubts before committing a lot of time and money to the partnership. Also make sure you retain all the rights you need to use the illustrator’s output in the ways you may need to, i.e. for further publication in other media, etc.