I third vrbo.com, or try homeaway.com. Most condos are on both websites, so it doesn't really matter; we've used both many times.
On the Big Island we usually stayed at a resort, but if you ever need a condo in Maui, I know of a few great ones! Keep in mind that there are less sandy beaches on the big island, a lot of them are lava rock, so there are less "beachfront" properties so to speak. They will still be oceanfront, just not on a sand beach, if that makes sense.
Have a great trip!