I've searched for an answer to my question but maybe I don't know what exactly I'm searching for. I'm wondering if there is a service in the US where I can buy a SIM and a block of data and then use up that data over time instead of the data expiring by the end of the month or paying every month. I have a Nexus 7 with 3/4g and would like to occasionally be able to use a data connection when I don't have access to WiFi.

Am I right to think that this just doesn't exist here in the US? Thanks for any information.