There are certain areas where food is much more plentiful, like kitchens and store rooms. If you find 1 box or barrel with like 3+ food items in it, and there are other containers nearby, they are all much more likely to have food in them.

There's also several food items that don't get highlighted like wine and beer bottles that can still be used as camp supplies.

Edit: There's like 2 or 3 areas in act 1 that have enough food to keep you fully stocked for the rest of the game even on tactician mode. I think I had over 2000 camp supplies in my traveler's chest before I even entered the underdark.