Suggestion for mobile plan in the US

Dear readers, my girlfriend stays for 3 weeks in Florida, and I thought to overcome the expensive roaming costs, it could worth to buy a cheap phone with a "prepay"- plan. (Not sure how it's called in the US. Please correct me, if I'm wrong) So, here my questions: -Is it required to live in the USA to create such a mobile plan with a carrier? -Do you have already some plans in mind? Any recommendations? -Are there currently some good offers available? The deal should be with a cheap device (Her regular phone is simlocked), to use some online services on the go (Specially map services) Thanks for your advices and experiences. cwa