Hey. Im stydying art at uni.
So hopefully I can help a little.
To locate a illistrator, maby look for artists in your area advertising in things like horse magazines and see if any of them are able to produce work for you.
If they can help you, You will have to work out away to work together, as in time scedule and such and sort out which parts are to be illistrated. Its proberly easier to let the artist choose, as they will be able to pick bits that best inspire them , so you will get a more fluent image rather than a forced one.
If they arnt able to help you in what you need try going into galleries in your area, ask any gallery owners about hanging work that catches your eye, tell them your situation and they may be able to help direct you to someone.