Yes, you can buy the ticket on spot. The ticket to Suzhou is not hart to get. Otherwise, you have two days in Shanghai and you have time to buy it, no matter in railway station or ticket office in downtown. Well, I still believe you can get the ticket while you are leaving for Suzhou. Suzhou is only one hour or so from Shanghai by bus (39 minutes by bullet train and 1 horus or so by other K or T trains). If you want to take a cab, it may cost300-400RMB. So, I don't think it is a good option to take cab. If you miss the D trains, you can take T or K trains, or bus.