Run don't walk to the Wukesong camera mall in Beijing.
Pretty much everything there, really. Lots of second hand places, brand new shops and accessories too. I got an inexpensive tripod I really liked there, even some film/paper/chemistry places too.
Not sure when it happens, but when I was there last fall on a Sunday (IIRC) there were tables set up all around the large building, and they had a flea market kind of thing going...just fantastic. Seems like a regular thing but I don't know if it's weekly, monthly or what.
Accessible easily from subways...just get off at the, you guessed it, Wukesong stop. Not hard to find.