Help Europe sim card questions


I am in Canada and I have an IS. I spoke with my carrier to get it unlocked, and they will do it for a charge. I am making my first trip to Paris, and then Rome in two weeks.

- What do I look for in a sim card?

Carrier, connectivity, rates? I don't know anyone there so I may only have several local based calls. I will need to call back to Canada quite often.

Can you buy a prepaid international card?
Where do I buy the card? - Internet, airport, corner store, dude on the street?
What sort of rates are available?
What carrier would be the best?

I am lost on this part of the trip, so answers will help.

Thank you.