What is your definition of healthy? I know a lot of restaurants, ethnic in particular, that claim their food is very healthy, but that is in comparison to what it could be. It is still pretty heavy on refined carbs and oils, but you could pick and choose the more healthy items and work the restaurant. Same with a lot of delis.
Another choice is to work with a caterer. I odn't have as much experience with that.
Number of people? Dinner or apps? Cost per person you are targeting? All of these will help you get better suggestions.