For clothing stores, I think the chains have all been mentioned, Addition-Elle, Rietmans, Laura II (plus), Cotton Ginny Plus, maybe Antels? The large size department at The Bay (La Baie) store downtown is very good and carries the designer large sizes, Elizabeth etc.
There is a very good shoe store on St-Hubert that carries boots for wide calves, very expensive (over $200), but consider that they are full leather products, no cheap stuff. The kind of shoes and boots that you take to the shoemaker and have rebuilt endlessly. It is called Chaussures D'Antin, 6629 St-Hubert.
For fabric stores, since I'm already down on St-Hubert checking out the shoe stores, I go over to C & M Textiles, 7500 St-Hubert. It is the kind of fabric store that has everything, there is lots to browse and lots of help - French speaking - but they have a huge warehouse of everything else, if you have something in mind, ask for it. There are other stores in that area to check out also, but this one has been there forever. They also have a parking lot! It doesn't hurt to check out the chain stores also, BouClair is very established and popular.
Roberta