In July and Aug you most likely can't; those places are snapped up quickly. I just experienced it last week. All reasonable Airbnb accomodation sold out - only the very expensive places left (EUR 150+) - No thank you.

Yes, I'm aware that private rooms/apartments is a big thing in Croatia; the blue signs are allmost on every house/building in Zadar, Trogir, Split, Hvar etc.