It depends on what you are looking for but I think a range of food options (instead of just full serve restos) is a great way to go, particularly if you have a kitchenette.
Great suggestions around Costco, happy hour, food trucks, and markets. I love to track down a fish store to buy poke or some fish to make at our rental. I also love tracking down a farmer's market to buy fruit or a snack at a prepared food vendor.
The single $ places can be pretty delicious but you're looking at counter service instead of being waited on.
Similar to other travel destinations, we try to get a variety of recommendations from recent travel articles, travel articles from online newspapers, youtube videos, blogs, etc. For Hawaii, we also try to get the corresponding book from the Hawaii Revealed series which I find has good food suggestions along with all the typical info you'd find in a travel book.
Some budget suggestions via our notes from previous trips to research if they meet your tastes. I did a cursory check to see if they are still open:
Oahu
Kua Aina - Delicious fish sandwich
Matsumoto Shave Ice
L&L Hawaiian Barbeque (various) - Chain
Big Island
South Kona Fruit Stand
Da Fish Place Lunch Wagon
Da Poke Shack
Anuenue Shave Ice
Scandinavian Shave Ice
Killer Tacos - kalua pig tacos
Tex Drive-In - malasadas
Kona Island Brewing - For the beer. Food meh.