Where can I buy the ticket? on the spot on the same day? Should I buy it beforehand?

No need beforehand because the reservation is for travel agencies. In fact, it is easy to get the ticket when you get to Yangshuo, such as the hotel, local travel agency offices, supermarkets, etc. The ticket is around 200RMB. If some tell you that you can just pay 40 or 50RMB, do not believe them.