There is a street with camera shops on both sides, many selling new stuff but also used cameras/lenses and accessories. I forgot the name of the street, but it is near one end of the river. I found it a very convenient place, and many shops give good service.......you need to bargain abit off course. One shop sells many pro used stuff, so be selective. Others have smaller selections, but you need to test them as usual.
I found the service very good, no need to feel afraid. Definitely better service than in some Orchard or Sim Lim shops! Some shops are big, and service is something like Cathay. You do need to speak mandarin, english not so good there, but I always had no problems using my hands
Hong Sien