I remember Samantha stopping in a store in Paris that sold designer clothes that were inexpensive because they were the previous season's fashions (I think). Anyways, I'm taking my family to Paris for Christmas & would like to take them (wife & two daughters) to that store.
The big (= expensive) couture houses, such as Givenchy, Yves St. Laurent, etc., are near the Louvre. If one walks from the obelisk where the guillotine stood toward the Louvre, they're to the left (north).
On the other hand, if one is looking for custom designer clothes, the Montmartre area - particularly the area just down the hill - there are many houses that will custom design and fit for a very reasonable price. I recently feted my daughter all over the city for this purpose and became an unwitting expert in the field.
To pass on some valuable information to the younger crowd (as is my 24 year-old daughter), Paris couture is for those over 30. There is very little the younger folks will enjoy. For the younger crowd, London is the fashion capitol. Just be forewarned.