Aloha, for store specifics that you can see on a map, or on a list by type and distance, this is a great website: http://www.MyHawaiiHomePage.com.
To find information about stores, restaurants, attractions/recreation, banks, etc. in a given area, go to http://www.MyHawaiiHomePage.com. Then, 1) in the upper right corner Quick Search box, type Diamond Head (or an address or landmark in the area). 2) From the property list that appears, select any property and click Details. 3) Then from the Propety Details page, click Neighborhood for demographics. 4) From the Neighborhood page, click Neary Establishments. They will show up as a list, or if you prefer a map, click on More Details/Map.
To use the search, you'll be asked to select an agent -- please click on my name. (Your information won't be sold and my company respects your privacy.)
Also, to add to Chery's notes, there's a City Mill hardware/ lumber/ paint store at the corner of Waialae Ave. & St. Louis Heights Dr. It's smaller than Lowes & Home Depot but has a lot of basics for home improvement. Thanks and good luck!