People report buying them at Relay shops at CDG.
If you don't want to mess with setting up a SIM upon arrival and want a working phone when you land in Paris, consider buying a SIM ahead of time like I did - from countries like the UK and the Netherlands that don't require registering with a passport to activate one. Here's my Dutch Vodafone experience:
If you are leaving in less than two weeks, though, might be too late to order one.
FYI, careful with your AT&T phone - AT&T locks their phones though they will unlock them upon request if the phone has been paid off. If you've never unlocked your phone, I'd go to AT&T's site now and request an unlock code. You need to enter that code only once, after you have inserted a SIM that wants to use a non-AT&T network; enter the unlock code the first time and if successful, that's it. The code doesn't expire and would be the same now or in six months if you request one.