I have had good luck at Le Poilu for pre-1945 items, but prices may seem high by North American standards. For more current French and Euro army stuff, I agree with Bousquet - the flea market at the Porte de Glignacourt is always good. There is one area in the market with lots of surplus stalls, the largest of which is Tranchee Militaire: https://www.trancheemilitaire.com/
The nice thing about the flea market is that you can cover lots of stuff in one area on foot (although the market is large, it can still be covered on foot). The other surplus shops in town are spread all over the place, and for the time it takes to reach them on the metro, I find it is better to spend the time in the market. Also, I found the prices there to be more reasonable than shops like Doursoux.
If you go to the market, bring a large backpack to carry any finds, otherwise it can get pretty exhausting.