Very good option is http://www.lefrenchmobile.fr/ They have a pre-paid card with a reasonable price. The good thing is that you don't have to sign any contract, give them any account number or whatever. You just buy a SIM card and a credit in their e-shop.
You can have the card shipped all around the world. It is activated once you make your first call, and at that moment you are assigned a phone number, too. Once you don't want it, you simply drop it. They diminish EUR 1.40 every month from it if you don't use it, and once you have no credit left, the account is terminated.
I have it and I'm really content with it. All service is in English, the delivery (inside Paris) was within couple days.
I know this is not a "buy near Arc de Triomphe" option, but still, it is valid as an answer for future visitors of the question.
Another option is the operator http://free.fr/ that launched their first shop at 8 Rue de la Ville-l'Évêque, 75008 Paris in the North-Center Paris, close to many metro stations (lines 8,9,12,14 and others). It has become very popular and they have tariffs as cheap as EUR 2 a month.