Sim card for my iphone!

Helloooo
i will be visiting your beautiful country soon, my travel will start in buffalo all the way down to miami.
Since i will stay in the U.S. for 23 days i need a sim card of an american phone company to put in my iphone replacing my italian one, in order to avoid thousands euros of international charges.
I'm not interested much in phone calls, the thing i need the most is the data connection.
Do you have in the U.S. sim cards which does not require subscribing any contract?
I mean somenthing prepaid which has unlimited data connection?
Here in italy i pay 9 eur per month and they give me 1 gb of internet connection at high speed, and after that they reduce my speed up to the next month.
Does something like that exist in America?
Could you please suggest me which company is the best and what i have to ask for?
thank youuuuu
Giovanni