Axel, 4-700E can get you pretty far in Paris.
My two recommendations are:
Boggi Paris (they just opened about a year ago)
112 Boulevard Saint Germain
Bruce Field (this is a chain store there are about 15 of them all over Paris) -- they are like the French budget Brooks Brothers, price is a tad lower than Boggi, but you can find decent suits and jackets there.
There is a Bruce Field store next to Boggi (136, Bd Saint Germain), and there are a couple near l'Opera and Comedie Francaise.
In fact, if get off at subway St. Germain des Pres and walk to the east along blvd St. Germain, there are a number of shops worth visiting until you reach blvd St. Michel. It's about half a mile long. (Check out Loding Shoes and Finsbury Shoes.)