a long the dalmatian coast there i plenty of small camps sites which you do not find on the internet, if you are lucky they might be free
On most of the island, there is limited number of campsites, so it might be a bit harder to find something here.
Generally it is hard to say how July and August will be here, up front bookings are down, but it is hard to say how many people will deside to go here last minute.