there's a building next to shanghai's north train station that sells both new and used camera gear, both modern and vintage. the bad thing is everything there, especially the vintage camera and lenses, are all overpriced. you can get it for 20% cheaper on ebay in the U.S.
hongkong is a different story, they have a lot of nice used camera shops. look up ebay auctions, especially for contax zeiss lenses. a lot of sellers are from hk and are stores in hk. prices are a bit cheaper than U.S.
when buying from asia, especially places like hk, you want to pay close attention to fungus in lens. the humid conditions have not been kind to unprotected lenses