Should ask me mah . I was in my JC and Sec school Chinese Orchestra troupe, and was a QM for 2 years... Best place I would say is Eason in Rochor Centre, I think it is on the second story. Do check around, the place is quite big, and the store is at a corner. They have cheap ones and expensive ones. They sell mainly chinese instruments of all sorts and also some books. Good luck!
PS. There is another place, but I kinda forgot since it has been more than 2 years since I touched CO stuff xD
TS,What he is referring to is a Music shop named Synwin @ Tanjong Katong Shopping Complex. Last floor.
All i know,The price might be tad high,but they have nice services.If your guzheng any prob,ring them up again and arrange a date to bring it down to them to check it out . =)