Just returned from Prague (my 25+'th trip down there)...
there are two big photostores down there - and they are in the same street, so easy to find.
Name of the street: Vodickova (sp?) - one is called FotoSkoda and the other a little further upthe street on the other side is called Fotooptika.
The used market is not what it used to be.... Too many rich americans/germans and japaneese people have almost vacuum cleaned the town... (not my words - words from people working in these stores) - but there are still fine things to see and maybe buy.
In any case - Prague is one of the most beautiful cities in the world - you're in for a treat!!