I want to buy an erhu, but I don't know where it is.

You'd better ask friends or teachers who can play erhu to help you choose and recommend erhu made by famous teachers such as Shanghai Dunhuang brand, Lu, Wan Qixing, Xu Xuru. Now the erhu produced in Beijing is also quite good. It turned out to be a pianist from Wang Guoxing who went to Beijing Musical Instrument Factory. It's half price in the music store.