In Croatia you'll never find the kind of room you're looking for online. You have to just show up. In bigger places like Dubrovnik you can walk into a travel agency and they'll help you find a room. In smaller places like Hvar there will be people meeting the boat (or bus) with offers of a room. They'll take you to the room and let you see it before you buy. Make sure it's in the center of town, not way out in the boonies (you should be able to walk there from the bus station or ferry dock). A lot of the people with rooms to rent are older "babushka women" and they often don't speak much (or any) English; German or Italian are much more common there. But it's also not difficult at all to learn enough simple phrases in Croatian to get you through a room rental.
Actually I never paid anything near 30 euros for a private room in Croatia; for one person it was more like 10-15 euros.
One caveat: although there will be people meeting the boat/bus in Dubrovnik with room offers, I would recommend taking a taxi or bus straight into the old town and finding a travel agency. Most of the people meeting the boat/bus live way out in the suburbs, not where you want to be.