The official Italian tourist board, a government body, runs a hotel booking service and has an office at the railway station of all Italy's major cities (the one in Rome, however, does not act for the cheapest hotels). If you tell them exactly what you are looking for they will try to best match your needs, then ring the hotel to book a room for you. Best of all it costs nothing; You pay a small fee which is set against your hotel bill. And you get a free map to help you find your way there, usually marked with the nearest vaporetto stop.
Watch out for the touts who accost you as you queue. They are not official.
Inside Venice railway station
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