There is no simple and easy to use availability calendar. There does seem to be calendars for a lot of other things but none that are simple and JUST show availability. I found a program called Availability Calendar (http://www.ajaxavailabilitycalendar.com) a PHP based program. However, it doesn't seem to work very well in WordPress. I did find an unworkable version plugin of this program but the designer is not responding to messages, I'm guessing he abandoned it.