Last time we were in France we just went into Carrefour (huge big-box store) and bought one off a rack in the electronics section. The card was from the carrier "Orange." When we put it in the phone all the instructions came up in French, but when we switched the language back to English it was easy--the phone number was there, and we were texting our kids back home a few minutes later. We ran out of money at some point but we were able to re-up the card remotely, IIRC.
Just got back from both. Didn't use my phone in Paris but once in Italy I got a Vodafone sime card. 30 euro with 30 euro credit. The bis is 3 euros a month and they take it off the card. Was not able to get the browser working but all other data services including email.