The musicians keep coming out of the woodwork. Your best bet is one of the shops on Xinjiekou Nandajie, although I only know of one or two that might even have anything like that.
Or...you check out this online shop, they have a pretty solid selection and their sales department can speak a little English: http://shop.6735.com/ add one of their sales people on MSN or email them, and they'll set you straight. Free delivery too! I think.
OR. You can check out this Taobao how-to guide, written just for Chinese-illiterate folks like you and me: http://taobaofieldguide.com/