First of all, there are not many options inside the park area. But don't worry, you will certainly not get hungry during your visit.
If you plan the all-day visit of the Lakes, you can buy food at entrances or take a rest in one of the nearby restaurants.
There are also few restaurants on station points and some lake-side options where you can get a snack or a full meal. We have a comprehensive list of restaurants, bistros and cafe bars in and around Plitvice Lakes here.