North Shore: Try Sterman's Realty. If they don't have anything, they can direct you to a good company.
Diamond Head: there is not one legal vacation rental in this area that would fit your needs. The only legal vacation rentals are on the Waikiki side of Diamond Head at the Diamond Head Beach Hotel. Be warned that the neighbors in Diamond Head are pretty assertive about calling the City Building Dept on illegal rentals. I live in the area & see it. Any agency offering Diamond Head rentals is offering an illegal rental.
you might also try vrbo dot com
We rented an oceanfront house from them this past July. It was on north shore next to Waimea Bay.
Not sure if it was "legal" or not, but will be sure to check legality WHEN (not if) we return.