I have not visited all these shops yet, so I can't give you the most complete advice yet. I will update this posting after I've had my fill of bargain hunting in Paris.
My impression is that most people are unaware of the existence of these stores. But I think it's worth a peek. You never know what treasure you may find in all that junk.
Strolling down Avenue de Clichy in the seventeenth arrondissement, armed with some addresses off the internet, I found two or three Guerrisols. They each sell used clothing (beware, some looked very used, well-worn) at 1, 3, 5, 10 euros, and some special items for more. I found a lucky pair of purplish and stylish synthetic snakeskin boots in excellent condition for only 5 euros. Oh yeah. If you want boots, they've got millions of pairs! I don't know where they came from! The store at 19 Avenue de Clichy is large. Clothing didn't look too great to me. There were supposed to be some more Guerrisols at 29 and 31 Avenue de Clichy, but I didn't see them. I kept walking and found the last Guerrisol on the street after the 50's. That one's not really worth going into -- very small and I think has mostly men's clothing, except for the frumpy polyester dresses in the back.
Avenue de Clichy has many inexpensive clothing stores -- one side of the avenue seems reserved for men's clothing mainly. Start walking from the metro stop at Place de Clichy.
After exploring the Pigalle area, I discovered another Guerrisol which I haven't seen anywhere on the internet. It's at Place de Rochechouart, the intersection of Boulevard Rochechouart and Rue de Rochechouart. This is probably the biggest Guerrisol, with a mind-boggling amount of denim (including Levi's) and more boots. They had a scary bin of weary aprons and another with very sad children's wear, but if you're looking for coats, there are plenty upstairs. Same prices as the other stores. Closest metro is Barbes Rochechouart (18th arrondissement).
Supposedly, there's another Guerrisol at 7 rue Marx Dormoy in the 18th arrondissement near metro La Chapelle, next to a pawn shop. Haven't checked it out yet to see if it's still in existence.
I believe the Guerrisols are open Monday to Saturday, 10 to 7.
From what I've read, Guerrida is the trendier, more expensive sister store of Guerrisol. I'll check it out soon.
47 avenue de l'Opéra
01 44 56 00 73
Used coats in the Marais (4th arrondissement)
I haven't been to this place yet -- don't even know the name or address. Can't find it online. I just know there is a used clothing store that has mostly coats priced at ten euros. The street: Croix de la Bretonnerie. Metro: Hotel de Ville.
This place is supposed to have the good stuff. I haven't been there yet but have heard and read good things. It's in the 16th arrondissement, where many affluent Parisians reside, so you might luck out and get some designer item for three or five euros. They're only open Monday to Friday, 2:30 to 6, and the first Saturday of the month from September to mid-July. Closed in August. Get there before 2:30 to get the best pick. (40 rue La Fontaine, metro Jasmin)