It's not that hard. Like everyone says just post up a flier that says you're having a casting in a college. If you're to old then get a student that you know to do castings for you and record it. Then you pick the girl you want and offer her $50 just to say what you want.
Average college girl doesn't think like we do. To her $50 is like 1 handle of 40 and 6 tequila shots.