The easiest and cheapest way to do this, afaik, is to get, and use, a mifi - this is portable wifi, basically. It needs its own sim card, but you get a data only plan, iyswim, and you can use a French data-only sim if you want.
If you want to put a French sim in your iPhone, well first, you'll have to ensure your phone isn't locked to 3. Second, you'll need to get a mini-sim in France. I don't know how hard it is to get PAYG sims there - it seems to vary from country to country. This page is probably useful.
If you can get your phone unlocked, the orange payg mobicarte is good but its a bit of a faff to set up (google "mobicarte option internet max") gives you a months data for 9 but doesn't include email (but you can use webmail instead). Also it can eat your credit in seconds if you havent activated it properly.