Hi Manu,
Here are some adresses :
*LE POILU,18 rue Emile duclaux,75015 PARIS.
Metro : Pasteur (closed on Mondays)
They may have uniforms but for sure medals and badges and even reference books.
*DOURSOUX,3-5 passage Alexandre,75015 PARIS.
Metro : Montparnasse or Pasteur.
They may have some soviet stuff but quite pricey.
http://www.doursoux.com/
*EUROPSURPLUS,2 Boulevards Voltaire,75011 PARIS.
They may have soviet stuff and deac weapons.(If I remember well,closed on Mondays).
http://www.europsurplus.com/
For reference books...you must at least visit if not both,one of these 2 bookstores :
*La Librairie du collectionneur,15 rue de Cronstadt,75015 Paris.
Open Monday thru Friday,10h to 19h.
http://www.lehussard-librairie.com/
*Armes et collections,19 Avenue de la République,75011 PARIS.
Open Monday 12h-19H,Tuesday-Saturday 10h-19h.
http://www.librairie-ac.com/accueil.html
Otherwise,needless to go to the flea markets at St Ouen or Montreuil : overtouristized,then logically overpriced.
Hope this will help,have a good time.
Cheers,JP.