I think the options stated above are all pretty good ones. The Wal Mart doesn't really have a large grocery section though. The Foodland at Ala Moana is small and expensive. Don Quijote also sells electronics and other stuff (clothing etc) in addition to grocery and produce. There is also a nice food court outside. I find the other retailers (Safeway and Times) have raised prices. Don Quijote also sells really affordable angus beef. I think they had cheap persimmons a few weeks ago too.

I think Costco by bus is doable if you're not buying that much. Otherwise you'll have to lug a lot of groceries through a parking lot and cross the street to the bus stop.