Your trip sounds wonderful. www.kamera.ru is reasonably central in Moscow, a couple of kilometers from Kiev Station. The entrance is on a back yard, downstairs. The website lists current prices, so check it out to see if its worth the trip. I am sure there are other places as well. The real bargains, however, are when people are selling gear through internet ads.
Izmailovski Market in Moscow can be fun. You will find all the typical russian souvenirs, fur hats, soviet memorabilia, fine rugs from central asia, cheap DVD:s and also some stalls with camera equipment. It is rather touristic and beware of the fake wehrmacht-leicas. You will find all kinds of cameras and lenses. Saturday or Sunday mornings are the best.