Camping supplies and fuel (if you don't trust your firemaking abilities, I failed in that respect on the Rock Islands) are readily available in Koror. Check out the second floor of the Surangel Department store.
The supermarkets of Koror (WCTC and Surangel downtown are the biggest ones) have a wide selection of products. There is one tiny little shop on Kayangel which should only be considered as a last resort (though you could stop by for a soda and snacks for the boat ride back).
