Hi Matus,
If ever your stay is during until the June 5th-6th weekend, there is one of the most famous photographic fair of France, at Bievres, in the west suburb of Paris (can be reached by train), the whole center of this big village is reserved for cameras and related sellers !
Shops on boulevard Beaumarchais seem to be fewer and fewer each years, to my grief.
Some other good addresses, I hope they are still open :
* Photo Odeon, was on boulevard St-Germain until 2006 but now only 73 bd Beaumarchais
* Mac Mahon Photo : 31, Mac Mahon avenue (Underground station "Etoile")
site here...
* Photo Muller : 17, rue des Plantes 75014 (Underground : "Alesia" or "Pernety") : a smaller shop than the previous, a little on the "esoteric" side, some East European rarities, driven by a helpful gentleman. As I cannot found any working website, I don't know if ever is he now retired. [EDIT] : I don't know when the google "streetview" of rue des Plantes was last updated, but Photo Muller seems to be still open !!!
If you decipher French, here is a list of sellers on Paris, funny but a little outdated (2008), I' afraid :
http://www.dmin-dmax.fr/photo8.htm
Hope this helps,
Regards,
Raphael