Here's a few links.

<a href="http://www.islandrentals.net/" target="_blank">http://www.islandrentals.net/</A>
<a href="http://www.sunsethomes.net/" target="_blank">http://www.sunsethomes.net/</A>
<a href="http://www.sandsea.com/" target="_blank">http://www.sandsea.com/</A>
<a href="http://www.firsthawaiianvacation.com/home.htm" target="_blank">http://www.firsthawaiianvacation.com/home.htm</A>

These should get you started. Most of them are realtors who rent out these places for their owners. Don't worry there's no time-share or other presentation. They just take a commission out of your rental price for their work.

You also might try www.asuguide.com it seems that they had some private condos on various islands at one time. I haven't checked them in a while.

Good luck, if you need more links I have them.