We're going to have 2 in diapers for our trip to Maui next month. I'd like to pack a couple of days worth of diapers, but buy the week's worth once I get to Maui (to save room in the suitcases).
Where is the best place for this?
The boxes at Costco will be too big for the week we're there.

Is there a Walmart or Target in Maui?

There's a huge Kmart right next to Costco (Kahului).

Oh, and a Walmart near Dairy Road (Kahului).

Excellent! Thank you.

Nice Safeway store in Kihei.

And another Safeway in Lahaina.

I wouldn't worry about taking diapers to Hawaii in your luggage. You'll want the extra space in the luggage anyway to bring back souvenirs.