But I actually never take the chance of "stumbling" on one. I make sure I know exactly where the action is by checking this website ahead of time (brocabroc.fr) and then on that day, make sure I get there even if it's way across town.
The people who set up stalls in a brocante are professional dealers. There are different associations of brocanteurs and you can eventually tell if the members of one particular association sells your type of stuff.
Sometimes I find something, many times I don't. But it doesn't matter.
Just seeing the items they are selling is educational in itself--the brands of porcelain, the proportion of the furniture, the types of items they sell and how much they are valued--all interesting information.
Where else but in Paris can you be looking at beautiful vintage things, and then look up and be equally enthralled by the architecture?
The teacups are always tempting...
The books are also interesting, but the ones in English usually cost much more than they should.
It seems this dealer hasn't sold this mirror yet, and it's been a year.
These crystal glasses were beautiful, but too much of a hassle to lug home.
I left empty-handed this time-- but still very much contented.