There are several No-COntract air-cards. Card runs 150-250 up front then you can buy bytes as you want....
I believe T-mobile has one.. I hear Verizon is going that way.. Straight Talk
The one that hangs in my bedroom is Virgin Mobile (NOTE I DO NOT recommend but well the story follows)
You need only ask at the cell phone store.
The story.
Where I usually winter one park had decent wi-fi, the other None (I split my time between the two) and I used about 20 dollars worth of bandwidth a month with the Virgin Mobile plan
It was kind of neat walking into a store and asking for a "Twenty Dollar Virgin".