If you're looking to sell your novel as a screenplay you're going to probably have to write that screenplay yourself. Most film agents aren't going to accept a novel, and most agents who rep novels will only rep ones that are already signed to another agent at their house. My advice is to look for a straight up literary agent based on your novel. Frankly, if you've got a novel coming out then you shouldn't have trouble finding an agent who'll want to get 15% for doing not very much!