Sim In France

[Bonjour_Voyageurs]

I also have read and read so many postings in many different forums but am still confused. Perhaps if I list my intention, someone can clarify or rectify me?
Itinerary:
My family and I are flying to Paris in May.
We are renting an apt. for 6 days.
Upon our arrival at CDG we are to call the rental agency IMMEDIATELY.
After 6 days, we are taking the TGV to Avignon, renting a car and driving around Provence for a week. We will turn in our car in Aix en Provence then a take train one day to Lyon, then Beaune, then back to Paris where we will stay one night in a hotel before returning home.
Phone idea:
Buy unlocked GSM phone on eBay.
Buy French SIM card where? In US so that I can give phone number to relatives or in France is the only option? If the latter is the only option, will I be able to buy the card and activate the phone at CDG so that I may call the apt. rental agency?
Many thanks for your guidance!
Michele