Gilles is, of course, right. He does have a job to do to earn a living, and the closer the race gets, the busier he is.
Stroll into Carrefour, find the phone section - normally near the entrance, but it's near all the computers etc. you need a portable sans abonnement - if you cant say it, write it down.
As a guide, I bought a Motorola with an SFR prepaid card 5 years ago for 500FF - 50 quid. It still works fine. You can re-chip the card back in the UK.