Peninsula is your best bet because there are a few shops there.
There is also Fotographics in Chinatown Point.
They sell alot of old stuff but you need to call the owner because his business hours depends on this mood. - 97831271.
Also, there are some shops in Albert Complex opposite Sim Lim Square that sell old camera stuff too.
Btw, my info is a little outdated so if there are changes, do correct me.