There are rental companies that have availability tools on their websites. Dvcrentalstore.com has one. However, those tools are not in real time. They are typically updated once or twice per day. So, it might give you a glimpse at what might be available, until you are ready to book they really are not that useful. Only owners can check availability in real time.
And FWIW, if an availability tool on one website says that nothing is available for your dates and preferred resort, that means that it's not available for any member to book, whether thru that rental agency or another or thru a private owner.