As long as we're talking about the same "marche au puces"
Yes there are 2 "gaming shops" one is run by an old man and you can ask him but I'm not sure he'll have. That one is in the "middle" of the place.
The one that is sure to have it is near the metropolitan entrance and has a lot of VCR movies and stuff for sale also along with video games. That guy is gonna have it. It not far from the black barber shop lol
Ps: all this is for the ds flashcart so you can play ds roms on your 3ds. However For 3ds "roms" just use cfw since a flashcard will make you take all the same steps as cfw so there is zero advantage. Also cfw will also allow any ds flashcart to work even blocked ones.