Where to find tourism information offices and kiosks in Venice
Venice's main tourist office is on Piazza San Marco, under the loggia in the southwest corner of Piazza San Marco (in other rods, at the far end of the square from the cathedral). It's on your left as you go under the loggia on your way to Salizada San Moisè.
Venice also has small (and very busy) satellite tourist offices at the Marco Polo Airport, the Piazzale Roma car parking garage, and inside the main train station, Santa Lucia (it's just before you lave the station, next to the left-hand exit doors; note that there are two miniscule offices side-by-side: one for tourism info the other a completely separate hotel booking stand, both usually with longish lines; the folks at the hotel office won't even give you a map of the city, let alone other tourist info, so be sure to get in the right line).
There's also an office in the mainland Mestre part of Venice at Via Lazzari 32.