Both the K-mart and the Wal-Mart on Kauai have quite a bit of food. They make me feel mainland tourist enough. I'd probably run into traffic if I went to Costco.

There are other alternatives to Costco. Safeway and probably others have loyalty cards you can get just for the asking, or perhaps you already have one from the mainland. The Long's food specials are usually very good deals.

My favorite grocery on the island is Sueokas in Koloa, but you would have to be staying south/southwest for that to be practical. Saves having to drag back through the Lihuie rush hour just for food (yes causal readers; I said rush hour). There are small IGA's up north that might be the same effect. Its kind of like shopping in the 1950's, but that is a good thing not a bad thing.

As much as it took to get there I try and be as island as possible and get the whole effect. For me that means local merchants, local dining, and local brands whenever possible. I'd rather sit on the beach with a box lunch from a parking lot shack than eat at Roy's. Sueoka's may be a bit more expensive, but not much, but its more fun and it keeps the island buzz going way better than Costco.