Another thing you can do is solicit an artist who has made artwork similar to what you are picturing for your book. Some artists can be found here on HubPages if you know what subjects to search for, but the best place is probably Deviant Art. It's a large community of artists that are familiar with this kind of stuff. Just make sure that you don't take any of their art without permission, and contact them to see what their rates are. There are a lot of high profile artists on that site, so some could have really high rates. Out of curiosity, what genre is your book?