I guess you should check out
www.pagesjaunes.fr which is a french e-phone book... just type luthier in ACTIVITE and paris in LOCALITE then you'll have a whole bunch of adresses, phone numbers, sites and plans available.
Anyway, most double-bass shops are located round the same area in paris that is "rue de rome". To get there, just take the subway, line 2 and stop at Rome station...
By the way, where will you play, with who etc.? I'd like to know, maybe i could get a chance to see you there...
Finally, you'd maybe like to check out this luthier in particular as he is supposed to be one of the best and most authentic one in France :Chauvelin James; 14 RUE CHARLES V PARIS.