I would Google the topic I have in mind and look thought the image search for a style I like. Then try to contact the designer to determine if they are interested in freelancing. Often freelance designers will have a website displaying samples of their work with their contact information.
I would also feel out their rates when contacting them. The cost will depend on the work you are looking for. You probably want to build in one or two revisions within the initial price.
I have worked with Patrick Wong before. He is very good and his rates are reasonable. www.patrick-wong.com
The following websites are places where some freelances post their work.