Where to get local SIM for phone?

The Relay shops at CDG have them and there are mobile phone stores all of over Paris. Most tabacs have them - just walk into the first one you come to and ask.

Make you life a lot easier and get a TMobile international plan. 20 cent calls and free data and texts.

The TMobile plan sounds much better than the AT&T one - that's what I have service with. I stopped in there today and the employees even discouraged me from buying their international plan.

Orange boutiques have SIM cards. Google the location nearest you and avoid going when the store just opens, at lunchtime and near closing time. This way, you'll have much better service. The staff will install the SIM, unlock the phone and make sure it works.
You might research Orange Holiday Plan online before you go. If it suits you, it's probably a better deal. Print out the pages and bring them to the store.

The Orange Holiday Plan SIM kit is available on Amazon. Good for two weeks after first use, 120 minutes, 1000 texts, and 1GB of mobile data. You could buy it long before your trip and be good to go when you land.

If you decide on Orange and arrive at Terminal 1: after passport and luggage pick-up, we followed the Exit signs (en route to the airport shuttle) and passed through an area that had a few shops, ATM, taxi stand outside, and an Information Desk. A few steps from Info was a Tabac stand that sold the Orange package. (We were directed there by representative at the Info Desk.) The young man running the shop was not helpful with installation, but we (decidedly non-techies in our early 70s) had no problem installing the SIM in an IPhone and launching it by following the included instruction pamphlet. I believe we paid something like E50 for a 30-day plan that included more than enough call-time and data for a 3-week stay. As others have noted, Orange storefronts seem to be everywhere in Paris.

I'm not sure I picked the best choice. I went to my hostel (Jules Ferry) in Paris and asked where I could pick up a SIM card, they pointed me about two blocks away to a little electronics store. I think the company I got was SRM (it was three letters anyway). The card came with some time and data (lasted about two days) then I had to go two more blocks to a bar to put more money on the card. I think I spent a total of 60 euro for the card and the phone/data for 17 nights. I used a lot of data searching for places, getting activity information and getting directions. I used a lot of phone calling lodging and a lady I met.

I spent about 50 euro total for two phones plus time for a month in Spain, but they were basic flip-phones with no data and I only used them to call ahead for lodging.

Jenny, I have used my TMobile from Istanbul through most of Eastern Europe and an awful lot of Western Europe and I have never, ever, had an issue. Just so convenient and with wifi calling using the unlimited data they provide or the hotel wifi; even the phone calls are free.

Thank you all for your suggestions! Unfortunately not enough time to order anything ahead of time, as we leave on Saturday - I'm kind of last minute here! But you've all given me really great suggestions, they all sound better than what AT&T has to offer.