Do you know any website where I can search and book buses?

Where are you going to? According to what I know, many bus stations don't provide online booking service. It's true that some bus stations provide online-booking service. However, they just have Chinese websites. Thus, I am afraid that it's difficult for you to book the bus tickets in advance. If possible, find a Chinese friend (if you have) to help you figure out if the bus stations you look for have websites. If they have, you might ask them to book the tickets for you.