buying sim card at CDG

You can purchase a SIM anywhere, at a convenience store (Relay is at the airport), tabac, electronics store (FNAC, Darty, Boulanger), or a big box outlet (Auchan, Carrefour, Leclerc, Giant). Orange has a booth selling SIMs/service at customs if you arrive in terminal 1, not sure about the terminal 2 options.

If you already have an unlocked GSM phone, all the options work.

How long the service will last or what type of data/talk time you have is a different matter. All the major operators; FREE, Orange, Bouygues, SFR, or one of the variants offer prepaid plans. These offers change constantly, but they are available. Just be sure you are not signing up for a 1 year commitment.

If you have a newer iPhone you can buy an eSim. Orange sells esims online:
https://travel.orange.com/en/offers/france/

Orange has 14 day and 28 day plans. You can top them up online (physical sim cards too) to add validity.

There are some less expensive esims around but I am not sure which network they use (I have a French plan so have not tried). Airalo has a 30 day sim card with 5 gb for $15, 10 gb for 23, 20 for 36.

You can see if your iphone supports esims by going to Settings>Cellular and looking for the esim section and "Add eSIM". Also check your carrier doesn't have your phone locked under Settings>General>About>Carrier locks > No SIM restrictions.

Otherwise, buy an Orange holiday sim at the airport. Orange has a boutique in the airport that I remember seeing a couple months ago but they also sell them in the Relay stores, saw an ad for it when I landed a week ago. Look for the "Buy here, orange holiday tourist sim card" signs. They have better service than some of the other prepaid sims you find in stores.

Thank you both. Orange seems to have 28 day plans, and as we will be in Europe for 30 days, we may need to go with Bouygues, who have 30- day plans. We will be in France, Spain and the UK.