Where can i get a sim card for my iphone for france?

I have an iPhone 5 and i will be traveling to France this summer for 10 days. I have read about where to get prepaid sim cards but they all seem to be bought in France. I want to know if I can buy it in the USA and then just put it in my phone. My phone is unlocked but my carrier does not support International. Also, when would i put it in my phone, the day i leave or could it still work while in America and I just put it in early?

I went to Paris with my old iPhone 5 last year and i just bought a sim card at one of the carriers there.