Just contact them and pay the fee. I've bought professional work from artists before (for various projects, including the Max B: LOAL).
Just make sure you say you're not looking to make money off the artwork. If you are, you better be offering them something in return (like exposure, etc., and be able to back it up too).
Make the email professional and respectful. However, dont act like a super fan. Instead, act like you're on the same level as them. They will respect that. If you lower yourself to them, they won't take you seriously. Treat them like you would treat anybody else. From my experience, "famous" people appreciate when you come to them as an equal (if he is in fact famous...I have no idea). Many people miss this important lesson and end up not being taken seriously.