If you don't go to Zhouzhuang during holidays and weekends, I think the bus will not be crowded. Actually, tour special bus at the Shanghai Travel Center. On the other hand, you could take a coach from Shanghai Passenger Transport Terminal in front of Shanghai Railway Station. The earlies bus leaves at 06:50 so that you could reach Zhouzhuang before 9am. From Bund, you could take the subway Line 1 to arrive at the bus station directly~