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According to Flora Tour (Twin City's Slovakian partner):
Selling points: the tickets in the FLORA TOUR Headquarters on Kúpeľná Street (Bratislava), in the FLORA TOUR Office in the Passenger Port (Bratislava) or in FLORA TOUR Office at Schwedenplatz (Vienna). In the Kiosk under Nový most (Bratislava) it is possible to make a reservation.
The exact location of these selling points is available on their website.